feat: add artifactapi_local_terraform resource type

New resource for creating local terraform registries in ArtifactAPI
(repo_type=local, package_type=terraform). These repos host providers
directly rather than proxying an upstream registry.

Schema is minimal: just name and description — no upstream-specific
fields like base_url, caching TTLs, or auth.

examples/resources/artifactapi_local_terraform/main.tf

@@ -0,0 +1,4 @@
+resource "artifactapi_local_terraform" "internal" {
+  name        = "tf-internal"
+  description = "Internal terraform provider registry"
+}