Make a MongoProcessInterface available to all stages, not just those that subclass DocumentSourceNeedsMongoProcessInterface

This would be a pure refactor without any functional changes, but will make it easier to do both SERVER-32088 and SERVER-29159. An initial attempt at resolving SERVER-32088 involved determining whether a collection was sharded at parse time, so we needed to use the MongoProcessInterface before execution time. This work should make that possible. Similarly, a proof-of-concept for SERVER-29159 needed to check if a namespace was sharded during the DocumentSourceLookUp::constraints() method, which similarly depends on this work.