If the Conduit client supports gzip, make calls with "Accept-Encoding: gzip"

Summary:
Ref T13507. Add "Accept-Encoding: gzip" to requests if we can decompress responses.

When we receive a compressed response, decompress it.

Test Plan: Added debugging code, ran some commands, saw smaller payloads over the wire and inline decompression.

Maniphest Tasks: T13507

Differential Revision: https://secure.phabricator.com/D21118