app/testpmd: support modify field flow action

app/testpmd: support modify field flow action

Add support for the RTE_FLOW_ACTION_MODIFY_FIELD to the testpmd.
Implement CLI to create the modify_field action and supply all the
needed parameters to modify an arbitrary packet field (as well as
mark, tag or metadata) with data from another field or immediate
value.

Example of the flow is the following:
flow create 0 egress group 1 pattern eth / ipv4 / udp / end
  actions modify_field op set dst_type tag dst_level 2 dst_offset 8
          src_type gtp_teid src_level 0 src_offset 0 width 16 / end

This flow copies 16 bits from the second Tag in the Tags array
into the outermost GTP TEID packet header field. 8 bits of the
Tag are skipped as indicated by the dst_offset action parameter.

op, dst_type, src_type and width are the mandatory parameters to
specify. Levels and offset are 0 by default if they are not
overridden by a user. The operation can be set, add or sub.

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