Add ability to update from a plugin's beta

[afragen]

Not certain if there's a standard for how to keep a beta on the dot org SVN. If there is this should be doable. If there's no standard, we would likely need to create one.

[afragen]

It should be possible to query the dot org plugins API and return an array of version for the plugin. We would then need to parse the array to find a beta and do a version_compare to the latest version.

https://api.wordpress.org/plugins/info/1.1/?action=plugin_information&request%5Bslug%5D=jetpack

1. We would need to search through all installed plugins.
2. Parse versions and check for beta tags
3. Update update_plugins transient with beta package

Would likely need to create settings page to check which plugins to be a beta tester for.

This method would rely on the individual plugin dev to properly create a tag with the beta.

[pbiron]

see https://core.trac.wordpress.org/ticket/49620

[afragen]

@pbiron I saw that.
I saw it. I think it would require large changes to the core update API as well as the plugin info API.

I could see it being a bit simpler to first write a plugin for handling that for plugins before considering merging into this plugin. I do have a passing familiarity with providing multiple offerings/updates for plugins. 🥳