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 results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14758