provide common dynamic git data to plugins

[cburroughs]

Is your feature request related to a problem? Please describe.

As described in https://www.pantsbuild.org/docs/tagging-docker-images#using-env-vars-to-include-dynamic-data-in-tags I'd like common dynamic git data like the sha or branch name to be available in BUILD files so they can be used to construct docker tags and other versioned fields.

Describe the solution you'd like

An object (git?) available with properties like commit_id or branch_name.

Describe alternatives you've considered Fork off a bunch of processes in a shell as described in https://github.com/pantsbuild/pants/discussions/17633

Additional context

• Slack thread: https://pantsbuild.slack.com/archives/C046T6T9U/p1678992567654969
• Some of this is in src/python/pants/vcs/git.py, but I couldn't figure out anyway to make that available in build files

[lilatomic]

we can expose symbols in BUILD files with def build_file_aliases(): in a register.py. I'm not sure if we can invoke rules within that context, though.