Allow *.search Conduit API methods to have data bulk-loaded by extensions

Summary:
Ref T11404. Currently, SearchEngineAttachments can bulk-load data but SearchEngineExtensions can not.

This leads to poor performance of custom fields. See T11404 for discussion.

This changes the API to support a bulk load + format pattern like the one Attachments use. The next change will use it to bulk-load custom field data.

Test Plan:

• Ran differential.query, differential.revision.search as a sanity check.
• No behavioral changes are expected
• See next revision.

Reviewers: yelirekim, chad

Reviewed By: chad

Maniphest Tasks: T11404

Differential Revision: https://secure.phabricator.com/D16350