net/ark: generalize meta data between FPGA and PMD

net/ark: generalize meta data between FPGA and PMD

In this commit we generalize the movement of user-specified
meta data between mbufs and FPGA AXIS tuser fields using
user-defined hook functions.

- Previous use of PMD dynfields are removed
- Remove emptied rte_pmd_ark.h
- Hook function added to ark_user_ext
- Add hook function calls in Rx and Tx paths
- Update guide with example of hook function use

Signed-off-by: Ed Czeck <ed.czeck@atomicrules.com>