net/mlx5e: Rearrange tc tunnel code in a modular way

Rearrange tc tunnel code so that it would be easy to add future tunnels:
 - Define tc tunnel object with the fields and callbacks that any
   tunnel must implement.
 - Define tc UDP tunnel object for UDP tunnels, such as VXLAN
 - Move each tunnel code (GRE, VXLAN) to its own separate file
 - Rewrite tc tunnel implementation in a general way - using only
   the objects and their callbacks.

Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>