codeintel: add support to gitserver to download artifacts from maven repositories

Created by: shrouxm

@unknwon still need to test everything, but wanted to ask for a review before i dive into that just to make sure the direction i'm going seems reasonable to you.

some things i'm punting on:

• i'm taking advantage of the logic we use for adding github/gitlab repos to dotcom to solve the problem of not pulling down all of maven at once. this logic was written as a special case, but is starting to look like it should get a proper abstraction. i didn't want to dive into a refactor when i'm so unfamiliar with the gitserver code though
• for VCSSyncer functions which return a command, i'm just allowing them to return nil to indicate that that functionality isn't supported for that syncer

@olafurpg the files vcs_syncer.go and coursier.go contain some logic that still needs to be filled in that you're better equipped to tackle than i. let me know if it seems manageable for you to dive straight into that or if you wanna pair/do some more knowledge transfer beforehand.

i'm going to leave this PR until i get a 👍 that the general direction is good and start working on a PR for #20835 (closed) since they're pretty much orthogonal

cc @efritz for context, don't have to review if you don't wanna though

closes #20834 (closed)