ethdev: introduce generic flow API

ethdev: introduce generic flow API

This new API supersedes all the legacy filter types described in
rte_eth_ctrl.h. It is slightly higher level and as a result relies more on
PMDs to process and validate flow rules.

Benefits:

- A unified API is easier to program for, applications do not have to be
  written for a specific filter type which may or may not be supported by
  the underlying device.

- The behavior of a flow rule is the same regardless of the underlying
  device, applications do not need to be aware of hardware quirks.

- Extensible by design, API/ABI breakage should rarely occur if at all.

- Documentation is self-standing, no need to look up elsewhere.

Existing filter types will be deprecated and removed in the near future.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Olga Shern <olgas@mellanox.com>