net/mlx5: Add shim layer between fs and cmd

The shim layer allows each namespace to define possibly different
functionality for add/delete/update commands. The shim layer
introduced here, will be used to support flow steering with the FPGA.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Aviad Yehezkel <aviadye@mellanox.com>
Signed-off-by: Boris Pismenny <borisp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>