mgmt: mcumgr: transport: shell: Add optional input expiration

Adds an optional feature that can be used to time out a partially
received MCUmgr packet over the shell interface. Prior to this
change, if a header was received, then the whole shell would be
unavailable until the module is rebooted or additional full MCUmgr
packet was sent.

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