Add GitHub CLI devcontainer feature

[Copilot]

Adds a new github-cli feature that installs the GitHub CLI (gh) in devcontainers, following the same patterns as the existing gitlab-cli and goreleaser features.

Changes

• features/src/github-cli/ — New feature implementation:

  • installer.go — Resolves versions via GitHub API tags, downloads gh_<version>_linux_<arch>.tar.gz from GitHub releases, extracts and installs to /usr/local/bin/gh; supports HandleGitHubOverride for corporate proxy/cache setups
  • install.sh — Shell entrypoint delegating to the compiled Go binary
  • devcontainer-feature.json — Feature metadata with version and downloadUrl options
  • NOTES.md / README.md — Compatibility and usage docs

• features/test/github-cli/ — Test suite covering Debian 11/12, Ubuntu 24.04, and Alpine

• override-all.env — Added GITHUB_CLI_DOWNLOAD_URL entry for per-feature URL override

Usage

"features": {
    "ghcr.io/postfinance/devcontainer-features/github-cli:1": {
        "version": "latest"
    }
}