feat: add Terraform/OpenTofu registry remote type (#45)

Implements the Terraform Registry Protocol as a proxy remote type so
Terraform and OpenTofu can pull providers through the caching layer
without changing provider source addresses.

- New `terraform` package type with `construct_url` (prepends
  `/v1/providers/`) and `resolve_content` (rewrites `download_url`,
  `shasums_url`, `shasums_signature_url` to route through a companion
  `releases_remote`)
- Built-in mutable pattern for provider version lists
  (`{ns}/{type}/versions`)
- `releases_remote` config option links the registry remote to a
  separate generic remote proxying the release CDN
- Client config: `.terraformrc` / `.tofurc` host block redirects
  `registry.terraform.io` to the proxy without touching `.tf` files
- 8 unit tests + end-to-end test (OpenTofu 1.10 pulling hashicorp/vault
  4.5.0 through docker-compose stack)
- Example config and README section added

tests/test_routes.py

@@ -1266,14 +1266,16 @@ class TestTerraformRemote:
     def test_download_info_download_url_rewritten(self, client, patched_deps):
         """download_url in download-info JSON is rewritten to point to the releases proxy."""
         deps = patched_deps
-        download_info = json.dumps({
-            "os": "linux",
-            "arch": "amd64",
-            "filename": "terraform-provider-vault_0.28.0_linux_amd64.zip",
-            "download_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_linux_amd64.zip",
-            "shasums_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS",
-            "shasums_signature_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS.sig",
-        }).encode()
+        download_info = json.dumps(
+            {
+                "os": "linux",
+                "arch": "amd64",
+                "filename": "terraform-provider-vault_0.28.0_linux_amd64.zip",
+                "download_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_linux_amd64.zip",
+                "shasums_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS",
+                "shasums_signature_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS.sig",
+            }
+        ).encode()
         deps["storage"].exists.return_value = True
         deps["storage"].download_object.return_value = download_info
         deps["cache"].is_mutable_file.return_value = False
@@ -1287,14 +1289,16 @@ class TestTerraformRemote:
     def test_download_info_shasums_url_rewritten(self, client, patched_deps):
         """shasums_url is also rewritten to the releases proxy."""
         deps = patched_deps
-        download_info = json.dumps({
-            "os": "linux",
-            "arch": "amd64",
-            "filename": "terraform-provider-vault_0.28.0_linux_amd64.zip",
-            "download_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_linux_amd64.zip",
-            "shasums_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS",
-            "shasums_signature_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS.sig",
-        }).encode()
+        download_info = json.dumps(
+            {
+                "os": "linux",
+                "arch": "amd64",
+                "filename": "terraform-provider-vault_0.28.0_linux_amd64.zip",
+                "download_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_linux_amd64.zip",
+                "shasums_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS",
+                "shasums_signature_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS.sig",
+            }
+        ).encode()
         deps["storage"].exists.return_value = True
         deps["storage"].download_object.return_value = download_info
         deps["cache"].is_mutable_file.return_value = False
@@ -1311,11 +1315,13 @@ class TestTerraformRemote:
         """The path portion of the upstream URL is preserved when rewriting."""
         deps = patched_deps
         zip_path = "/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_linux_amd64.zip"
-        download_info = json.dumps({
-            "download_url": f"https://releases.hashicorp.com{zip_path}",
-            "shasums_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS",
-            "shasums_signature_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS.sig",
-        }).encode()
+        download_info = json.dumps(
+            {
+                "download_url": f"https://releases.hashicorp.com{zip_path}",
+                "shasums_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS",
+                "shasums_signature_url": "https://releases.hashicorp.com/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_SHA256SUMS.sig",
+            }
+        ).encode()
         deps["storage"].exists.return_value = True
         deps["storage"].download_object.return_value = download_info
         deps["cache"].is_mutable_file.return_value = False
@@ -1332,7 +1338,9 @@ class TestTerraformRemote:
         deps["storage"].download_object.return_value = b"PK\x03\x04 zip bytes"
         deps["cache"].is_mutable_file.return_value = False
 
-        response = client.get("/api/v1/remote/hashicorp-releases-test/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_linux_amd64.zip")
+        response = client.get(
+            "/api/v1/remote/hashicorp-releases-test/terraform-provider-vault/0.28.0/terraform-provider-vault_0.28.0_linux_amd64.zip"
+        )
         assert response.status_code == 200
         assert response.headers["X-Artifact-Source"] == "cache"