R1:9499987cfe6f

Add "owners.search" Conduit API endpoint, with CustomField support

Summary:
Ref T9964. Adds a new-style "owners.search" endpoint, and an extension for customfields.

Puts enough indirection in place to give us nice, consistent "custom.key" user-facing keys instead of "std:custom:owners:na0shf9a8dfdsafl" junk.

Test Plan:
- Searched Owners via API.
- Searched by ID.
- Ordered by custom fields.
- Reviewed API docs.
- Used normal search with ordering.
- Viewed custom field values in search…