In Conduit responses, assert that Phabricator supports a "gzip" capability

Summary: Ref T13507. If we believe the server can accept "Content-Encoding: gzip" requests, make the claim in an "X-Conduit-Capabilities" header in responses. Clients can use request compression on subsequent requests.

Test Plan: See D21119 for the client piece.

Maniphest Tasks: T13507

Differential Revision: https://secure.phabricator.com/D21120