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
