msgpuck: activate MP_EXT custom serializers

Msgpuck functions mp_snprint() and mp_fprint() now support
customizable MP_EXT serializer. This patch makes them able to
correctly serialize all tarantool's MP_EXT extensions: MP_DECIMAL,
MP_UUID, MP_ERROR.

Closes #4719

Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Serge Petrenko <sergepetrenko@tarantool.org>
Reviewed-by: Nikita Pettik <korablev@tarantool.org>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>