mgmt/mcumgr: zcbor_bulk: Allow spaces in map keys

The CBOR encoding allows strings to have white spaces, and as string
may be used as key it should also be allowed to use space in key.
The commit provides ZCBOR_MAP_DECODE_KEY_DECODER macro, which is
intended to replace ZCBOR_MAP_DECODE_KEY_VAL macro, that allows
to use string keys with spaces in it.
Both macros are available for now.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>