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/conftest.py

@@ -118,6 +118,21 @@ TEST_REMOTES = {
             "immutable_patterns": [r"^v3/files/.*\.tar\.gz$"],
             "cache": {"immutable_ttl": 0, "mutable_ttl": 600},
         },
+        "terraform-registry-test": {
+            "base_url": "https://registry.terraform.io",
+            "package": "terraform",
+            "immutable_patterns": [
+                r"[^/]+/[^/]+/[^/]+/download/[^/]+/[^/]+$",
+            ],
+            "releases_remote": "hashicorp-releases-test",
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 300},
+        },
+        "hashicorp-releases-test": {
+            "base_url": "https://releases.hashicorp.com",
+            "package": "generic",
+            "immutable_patterns": [r".*\.zip$", r".*SHA256SUMS(\.sig)?$"],
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 0},
+        },
     },
     "locals": {
         "local-test": {