mgmt/mcumg/lib: Encode shell command execution result in additional field of response

[de-nordic]

Is your enhancement proposal related to a problem? Please describe.
Currently a code returned from execution of shell command is encoded as "rc" in response for a request; normally the "rc" field is used to encode error from mcumgr, so it may be sometimes not possible to distinguish between intended non-0 code from shell and mcumgr error.

Describe the solution you'd like
"rc" of the mcumgr/smp response should be only used to carry mcumgr errors; return code from shell should be given dedicated field in response.

Describe alternatives you've considered
Leaving as is.