Add Gerrit OAUTH plugin and setup for GitHub

Using v3.5.1 from
https://github.com/davido/gerrit-oauth-provider/releases/download/v3.5.1/gerrit-oauth-provider.jar
diff --git a/sourcearcade.yml b/sourcearcade.yml
index 2e3d279..ffb1279 100644
--- a/sourcearcade.yml
+++ b/sourcearcade.yml
@@ -46,6 +46,7 @@
       - ./gerrit/git/:/var/gerrit/git/:rw
       - ./gerrit/index/:/var/gerrit/index/:rw
       - ./gerrit/cache/:/var/gerrit/cache/:rw
+      - ./gerrit/gerrit-oauth-provider.jar:/var/gerrit/plugins/gerrit-oauth-provider.jar:ro
 networks:
   simpleidnet:
     driver: bridge