net/mlx5: support modify field flow action

net/mlx5: support modify field flow action

Add support for new MODIFY_FIELD action to the Mellanox PMD.
This is the generic API that allows to manipulate any packet
header field by copying data from another packet field or
mark, metadata, tag, or immediate value (or pointer to it).

Since the API is generic and covers a lot of action under its
umbrella it makes sense to implement all the mechanics gradually
in order to move to this API for any packet field manipulations
in the future. This is the first step of RTE flows consolidation.

The modify field RTE flow action supports three operations: set,
add and sub. This patch brings to live only the "set" operation.
Support is provided for any packet header field as well as
meta/tag/mark and immediate value can be used as a source.

There are few limitations for this first version of API support:
- encapsulation levels are not supported, just outermost header
can be manipulated for now.
- offsets can only be 4-bytes aligned: 32, 64 and 96 for IPv6.
- the special ITEM_START ID is not supported as we do not allow
to cross packet header field boundaries yet.

Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>