net/mlx5: E-switch, Move QoS related code to dedicated file

Move eswitch QoS related code into dedicated file. Provide eswitch API
to access this code meaning it is isolated and restricted to be used
only by eswitch.c. Exception is legacy NDO vf set rate, which moved to
esw/legacy.c.

Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Huy Nguyen <huyn@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>