Expand support for command extensions

Allows client libraries to have complete access to the rest of the
OSC ClientManager.  In addition, extension libraries can define
global options (for API version options/env vars) and define
versioned API entry points similar to the in-repo commands.

The changes to ClientManager exposed some issues in the existing
object api tests that needed to be cleaned up.

Change-Id: Ic9662edf34c5dd130a2f1a69d2454adefc1f8a95