Add the OAuth gerrit plugin to our mini collection of Gerrit plugins.
This includes a patch to make the plugin work correctly with CAS 6.x,
which has changed the attributes into a JSON object with the attributes
nested inside, instead of a JSON list.
Change-Id: I4741f137cca9c8eb45b9ea660fb4cbf6962be9a4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2782
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>