(stable) Allow construction of a ConduitEngine with a bare ConduitClient

Summary:
See PHI1735. "ConduitEngine" was once a future pool, but this has moved to "HardpointEngine". This class may no longer make much sense.

In Phacility code, "bin/host upload" depends on using the Uploader, which needs a "ConduitEngine", not a "ConduitClient". This workflow may use asymmetric key signing, which "ConduitEngine" does not support.

To unblock PHI1735, provide glue code between "Client" and "Engine". But a "more correct" change is probably removal of "Engine".

Test Plan:

• Ran bin/host upload, uploaded files (with additional changes to wrap the Client).
• Created this revision.

Differential Revision: https://secure.phabricator.com/D21260