ethdev: add flow API object converter

ethdev: add flow API object converter

rte_flow_copy() is bound to duplicate flow rule descriptions
(attributes, pattern and list of actions, all at once), however
applications sometimes need more flexibility, for instance the ability
to duplicate only one of the underlying objects (a single pattern item
or action) or retrieve other properties such as their names.

Instead of adding dedicated functions to handle each possible use case,
this patch introduces rte_flow_conv(), which supports any number of
object conversion operations in an extensible manner.

This patch re-implements rte_flow_copy() as a wrapper to
rte_flow_conv().

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>