Modularize SearchEngine extensions

Summary:
Ref T9964. ApplicationSearch currently has a bunch of hard-coded if ($object instanceof thing) stuff.

Pull that out so it can live in extensions.

Test Plan:

• Searched by spaces, subscribers, projects.

{F1023921}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14764