ethdev: introduce generic modify flow action

ethdev: introduce generic modify flow action

Implement the generic modify flow API to allow manipulations on
an arbitrary header field (as well as mark, metadata or tag) using
data from another field or a user-specified value.
This generic modify mechanism removes the necessity to implement
a separate RTE Flow action every time we need to modify a new packet
field in the future.

Supported operation are:
- set: copy data from source to destination.
- add: integer addition, stores the result in destination.
- sub: integer subtraction, stores the result in destination.

The field ID is used to specify the desired source/destination packet
field in order to simplify the API for various encapsulation models.
Specifying the packet field ID with the needed encapsulation level
is able to quickly get a packet field for any inner packet header.

Alternatively, the special ID (ITEM_START) can be used to point to
the very beginning of a packet. This ID in conjunction with the
offset parameter provides great flexibility to copy/modify any part of
a packet as needed.

The number of bits to use from a source as well as the offset can be
be specified to allow a partial copy or dividing a big packet field
into multiple small fields (e.g. copying 128 bits of IPv6 to 4 tags).

An immediate value (or a pointer to it) can be specified instead of the
level and the offset for the special FIELD_VALUE ID (or FIELD_POINTER).
Can be used as a source only.

Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>