Add release engine for WordPress

[ezekg]

With all the WP drama going on, I think providing a release engine for commercial WordPress plugins and themes makes sense. I personally don't know a lot about the internals of the WordPress update system, but I'm opening this issue to document. We should be able to create an engine that responds in WP's expected update format, whatever that may be.

Implement similar to #478. I'd prefer a wp subdomain, to avoid any trademark issues. See #679 for implementation of PyPI, and #753 for Tauri. The Tauri one will likely be the most helpful for reference, since the PyPI included a lot of noisy ground work.

Helpful resources:

• https://rudrastyh.com/wordpress/self-hosted-plugin-update.html

Other resources:

• Add a release engine for npm #489
• Add a release engine for Rubygems #491
• Add Rubygems release engine. #896

In the end, I want a customer to be able to upload a plugin or theme .zip to the distribution API we already have, and allow their end-users to get automatic updates for the plugin or theme like they would from the WP.org repo.

Implementation should include integration tests.