enhance: add slim version of git plugin

[wass3rw3rk]

in order to keep the base git plugin as lightweight as possible, we are splitting the plugin into two versions: vela-git and vela-git-slim.

vela-git will contain additional "helper" packages. at this point, that is gh and git-lfs, but there is potential it might grow further.

the -slim variant will only contain git - the bare minimum needed for regular git operations required for most "clone" steps. this will also be the default plugin for the go-vela/server and go-vela/cli modules (additional PRs will follow).

here's an incremental breakdown of size differences:

image	size
vela-git	30.3MB
vela-git + github-cli	82.9MB
vela-git + github-cli + git-lfs	90.2MB

when compressed, you are looking at roughly:

image	size
vela-git-slim (new with this PR)	10.6MB
vela-git (includes github-cli + git-lfs)	29.4MB

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.66%. Comparing base (612bfde) to head (c06cafc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #162   +/-   ##
=======================================
  Coverage   43.66%   43.66%           
=======================================
  Files           7        7           
  Lines         355      355           
=======================================
  Hits          155      155           
  Misses        186      186           
  Partials       14       14           

[KellyMerrick]

lgtm