R1:6e57582aff82

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.