subsys: mgmt: SMP protocol for mcumgr.

The Simple Management Protocol (SMP) is a basic protocol that sits on
top of mcumgr's mgmt layer.  This commit adds the functionality needed
to hook into mcumgr's SMP layer.

More information about SMP can be found at:
`ext/lib/mgmt/mcumgr/smp/include/smp/smp.h`.

Signed-off-by: Christopher Collins <ccollins@apache.org>