Add GitHub Packages (GHCR) support

[Copilot]

Adds support for backing up GitHub Container Registry package metadata, enabling backup of ghcr containers for con and other organizations.

Changes

• Config (config.py): New packages path option in GHPathsDict with gets_packages() predicate
• GitHub API (github.py):
  • Package, PackageVersion, ContainerMetadata models for API responses
  • GHPackageAsset class storing version metadata as JSON
  • get_packages() / get_package_versions() / get_package_assets() methods with org/user endpoint fallback
• CLI (__main__.py): Package fetching integrated into fetch command with DataLad support
• Docs (README.rst): New path placeholders: {package_name}, {package_type}, {version_id}, {version_name}, {tag}, {tags}

Example Config

repo: con/tinuous
ci:
  github:
    paths:
      packages: '{year}//{month}/{ci}/packages/{package_name}/{tag}/'

Output

Saves JSON metadata per package version (actual container images require docker/podman to pull):

{
  "package_name": "my-image",
  "package_type": "container",
  "version_id": 12345,
  "version_name": "sha256:abc123...",
  "tags": ["v1.0.0", "latest"],
  "updated_at": "2024-01-15T12:30:00+00:00"
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support GitHub Packages</issue_title>
<issue_description>Primary goal is backup of ghcr containers, for con and others ATM

May be could be done with some existing tool already.

attn @CodyCBakerPhD if you run into any existing one</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Support GitHub Packages #217

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