FR: Gerrit support

[ilyagr]

I'm creating a placeholder bug to reference.

See https://martinvonz.github.io/jj/latest/FAQ/#how-do-i-integrate-jujutsu-with-gerrit for current status.

This would likely be a collection of jj gerrit commands.

[steveklabnik]

See also #2845

[PhilipMetzger]

And also related #485.

[dbarnett]

What's the scope of Gerrit-specific functionality we're planning to integrate? From the links above it looks like gerrit send and/or something for rewriting commit messages on push, and then something for pushing stacked reviews.

I just filed a corresponding GitHub issue #4555 so I'm wondering what functionality would have Gerrit equivalents (and also whether jj [forgename] foo commands for those things are really necessary or should be modeled at another level).