mgmt: mcumgr: Replace mgmt_ctxt struct with smp_streamer

This replaces an intermediatory structure with a different one which
allows command functions to access the full contents of the streamer
structure that would be otherwise unavailable. This is a foundation
for allowing asynchronous mcumgr messages from the server.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>