net/sched: Add accessor functions to pedit keys for offloading drivers

HW drivers will use the header-type and command fields from the extended
keys, and some fields (e.g mask, val, offset) from the legacy keys.

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