Plugin Directory: Add a low-level upload-zip-to-svn api, basis for a github action

[dd32]

This is a work-in-progress, PR'ing it for sharing and future work.

This could be used by https://meta.trac.wordpress.org/ticket/6086 (Allow uploading ZIPs for plugin updates, to reduce friction) and as the underlying low-level basis for https://meta.trac.wordpress.org/ticket/6087 (Provide a GitHub integration).

This includes some things that will need to be changed before it can be used, such as the SVN auth, whether to overwrite trunk, whether those things should be an option or not, etc.

Please note: This PR existing is not a confirmation that it'll be merged anytime soon.