Merge requests: Search query builder refactor

Problem to solve

The code to build Elasticsearch queries is tightly coupled with the all ClassProxy code. It's too complicated to add new data models or modify existing models that already have a ClassProxy through a parent class (for example WorkItem)

Proposal

Use the general Builder library to build Elasticsearch queries for MergeRequestClassProxy. Remove all references to ApplicationClassProxy code when creating queries for merge requests documents.

Introduce behind a derisk feature flag