Beta: Allow non-WP.org plugins to have have stable versions installed

[tbradsha]

Proposed changes

When wanting to install plugins that are not in WP.org, there's no way to install the latest stable release.

This PR adds this functionality, pulling from GitHub (e.g. https://api.github.com/repos/Automattic/jetpack-beta/releases/latest).

The existing code is quite scattered and WP.org-centric. A refactor with generic objects populated from either source would be ideal, but that's out of scope.

Related product discussion/links

• beta: Improve plugin page interaction #48490 (comment)

Does this pull request change what data or activity we track or use?

Testing instructions

Ensure the stable release works for both source types.

[github-actions]

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