Add artifactapi_remote_github_alpine resource

Adds the metadata-only Alpine/apk analog of the github_deb/github_rpm
remotes: exposes a GitHub repo's release .apk assets as an apk repository
(synthesized APKINDEX) without precaching, redirecting downloads to a
backing releases_remote.

- register newRemoteResource("github_alpine") in provider Resources()
- add NewRemoteGitHubAlpine convenience ctor
- extend provider/metadata/ctor table tests + bump resource count to 22
- document github_alpine remote type in README
- add examples/resources/artifactapi_remote_github_alpine/main.tf

examples/resources/artifactapi_remote_github_alpine/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 .apk 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 = [
+    "goodtune/ghp/releases/download/.*\\.apk$",
+  ]
+}
+
+# Metadata-only remote: scans goodtune/ghp releases for .apk assets and serves a
+# synthesized apk repo (APKINDEX). Packages are never precached; downloads
+# 302-redirect to the "github" remote above.
+resource "artifactapi_remote_github_alpine" "goodtune_ghp" {
+  name            = "goodtune-ghp"
+  base_url        = "https://api.github.com/repos/goodtune/ghp"
+  description     = "goodtune/ghp GitHub releases as an apk repo"
+  releases_remote = artifactapi_remote_generic.github.name
+
+  mutable_ttl = 3600
+
+  # Optional: restrict which release assets become packages.
+  patterns = [
+    ".*_x86_64\\.apk$",
+    ".*_noarch\\.apk$",
+  ]
+
+  # Optional: token for private repos / higher GitHub API rate limits.
+  # password = var.github_token
+}