Add the skeleton for a "transaction.search" Conduit API method

Summary:
Ref T5873. See PHI14. This does the basics that are shared across everything (IDs, PHIDs, dates, comments).

It doesn't do types (I think I don't necessarily want to expose internal types over the API?) or transaction-specific data.

In the next change, I'm going to add ways to let ModularTransactions "opt-in" to providing more data to Conduit. I'll use this to flesh out the actual desired transaction types (comments, presumably inline comments) and likely leave the rest as skeletons for now until use cases arise so we don't create a backward compatibility issue (or a security issue!) by exposing tons of internal stuff as public-facing API.

Test Plan:
Ran queries, used paging. Retrieved an edited, deleted, and normal comment.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5873

Differential Revision: https://secure.phabricator.com/D18466