FR: Support Creation of Git Annotated Tags

[dbaileychess]

I still have to use a separate git repo to create annotated git tags, and would like the ability to make them via jj git tag -a. The lack of creation support is highlighted in the docs.

I think it makes sense as a jj git tag subcommand as it is primarily geared to git compatibility, as opposed to the JJ model. I would expect the annotated tags to continue to show up under jj tag list, as they currently do. Since annotated git tags are basically full commits (with no changes)

Additionally, the tag creation should be able to work on any change Id (e.g. jj git tag -a -c @ -m "blah blah" tag_label) or revision (e.g. jj git tag -a -r REV). I guess it should also be able to work on Bookmarks as well.