Gerrit sync

[KonradStanski]

This commit adds support for gerrit repo indexing.
It supports globs for both including and excluding repos.
It does not support branches or tags (as these are going to be reworked by brendan in the future.
It does not support auth yet (I'd need to spin up my own gerrit instance to test this out) - I can add this later hopefully.
Example URLs for experimentation:
// https://chromium-review.googlesource.com
// https://review.opendev.org
// https://android-review.googlesource.com

Sample config:

{
   "$schema": "./schemas/v2/index.json",
   "repos": [
      {
         "type": "gerrit",
         "url": "https://gerrit-review.googlesource.com/",
         "projects": [
            "apps/**",
            "gerrit**",
            // "modules/**",
            "zuul/**"
         ],
         "exclude": {
            "projects": [
               "gerrit-**"
            ]
         }
      }
   ]
}

[brendan-kellam]

LGTM 🎉🎉

[KonradStanski]

https://gerrit.wikimedia.org/
this is another example of a publicly available gerrit

[hashar]

https://gerrit.wikimedia.org/ this is another example of a publicly available gerrit

Hi, I am working for Wikimedia and the current maintainer of that Gerrit instance. Thank you for having added support for Gerrit 💯