RDMA/mlx5: Add a new flow action verb - modify header

Expose the ability to create a flow action which changes packet
headers. The data passed from userspace should be modify header actions as
defined by HW specification.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>