codeintel: Add RefDescriptions to gitserver client

Created by: efritz

Add a method to the codeintel gitserver client that will return the tip commit on every remote branch and tag for a repository. We will use this instead of only using HEAD to determine what indexes should be "protected" from eviction due to age.

Eventually, this will be called from the commit graph updater, which will also probably throw out a bunch of old tags and branches based on some configuration parameter (max age for branches so stale branches are not around forever).

Partial effort towards #19120 (closed).