feat: add artifactapi_remote_github_rpm resource

The artifactapi server gains a github_rpm remote type that exposes a GitHub
repo's releases as a metadata-only yum repository (synthesized repodata, no
precache, downloads redirected to a backend remote). Surface it through the
provider so these remotes are declarable as code alongside the other types.

- Register the github_rpm remote resource (artifactapi_remote_github_rpm),
  reusing the shared remoteResource plumbing; add the NewRemoteGitHubRPM
  constructor to match its siblings.
- Generalize the releases_remote attribute description: it now names the
  backend remote that serves download bytes for both the terraform remote
  (URL rewriting) and the github_rpm remote (302 redirect of .rpm downloads).
- Document the type and add an example wiring a github_rpm remote to a generic
  github.com releases_remote.

Tests cover the resource metadata type name, the constructor, and mapping of
package_type/base_url/releases_remote/patterns through the API model; the
resource-count and expected-types assertions are updated.

Depends on the github_rpm API surface in artifactapi (separate PR). Cut a
minor release (make minor) once merged.

examples/resources/artifactapi_remote_github_rpm/main.tf

@@ -0,0 +1,44 @@
+terraform {
+  required_providers {
+    artifactapi = {
+      source  = "git.unkin.net/unkin/artifactapi"
+      version = "0.0.1"
+    }
+  }
+}
+
+provider "artifactapi" {
+  endpoint = "https://artifactapi.example.com"
+}
+
+# Backend generic remote that serves the actual .rpm bytes from github.com.
+# Its patterns must allowlist the target repo's release-download assets.
+resource "artifactapi_remote_generic" "github" {
+  name     = "github"
+  base_url = "https://github.com"
+
+  patterns = [
+    "acme/tools/releases/download/.*\\.rpm$",
+  ]
+}
+
+# Metadata-only remote: scans acme/tools releases for .rpm assets and serves a
+# synthesized yum repo. Packages are never precached; downloads 302-redirect to
+# the "github" remote above.
+resource "artifactapi_remote_github_rpm" "acme_tools" {
+  name            = "acme-tools"
+  base_url        = "https://api.github.com/repos/acme/tools"
+  description     = "acme/tools GitHub releases as a yum repo"
+  releases_remote = artifactapi_remote_generic.github.name
+
+  mutable_ttl = 3600
+
+  # Optional: restrict which release assets become packages.
+  patterns = [
+    ".*\\.x86_64\\.rpm$",
+    ".*\\.noarch\\.rpm$",
+  ]
+
+  # Optional: token for private repos / higher GitHub API rate limits.
+  # password = var.github_token
+}