mgmt/mcumgr/lib: Remove cbor_encoder_writer from usage

With transition to zcbor, the cbor_encoder_writer structure of
TinyCBOR is no longer used.
This commit replaces the structure with cbor_nb_writer, which
gathers zcbor processing structure with netbuf into one object.

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