R1:2722c167d8c9

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…