Add a simple extension hook

This enables stand-alone modules to hook in to the command-line
handler in a dedicated API namespace.  These extensions have access
to the existing clients via the client manager but no way to add
another client to the manager is provided..

blueprint client-extensions

Change-Id: I4b2eaa780d8ba881db23d950d09b8702e9e314ef
Merge "Add a simple extension hook"