Support Ferret engine queries in ApplicationSearch via extension instead of hard-code

Summary: Ref T12819. Uses an extension rather than hard-coding support into Maniphest.

Test Plan: Saw "Query" field appear in Differential, which also implements the interface and has support. Used field in both applications.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18547