Add committer support in the query language

[morucci]

Currently we support an "author" field that is a representation of a Change (PR, MR, Review). However, we also index the committer identity (each commit in a change get a committer field). A committer is someone that pushed code in an already created Change like a new PS in Gerrit or new commit in a PR or MR.

Beside of the ability to filter on this new field, we could provide metrics about how a contributor is able to jump in Changes of other to make them move forward.

DOD: add the committer support in the Query language.

Thanks @fserucas for the idea