V2: add plugin API for VCS adapters #35162

Git and Mercurial support are now in for the current VCS surface.

The next VCS-adapter work is a plugin API so VCS backends can be registered by plugins instead of requiring a core fork. This should support systems like jj on top of the adapter seam added for V2 VCS APIs.

Source: https://discord.com/channels/1391832426048651334/1520962938733723720/1522589081282937052

Related:

• [FEATURE]: Expose V2 VCS status and diff APIs #34433
• [FEATURE]: Expose V2 session.diff API #34434