net/mlx5: Introduce modify header structures, commands and steering action definitions

Add the definitions related to creation/deletion of a modify header
context and the modify header steering action which are used for HW
packet header modify (re-write) as part of steering. Add as well the
modify header id into two intermediate structs and set it to the FTE.

Note that as the push/pop vlan steering actions are emulated by the
ewitch management code, we're not breaking any compatibility while
changing their values to make room for the modify header action which
is not emulated and whose value is part of the FW API. The new bit
values for the emulated actions are at the end of the possible range.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>