common/sfc_efx/base: support adding encap action to a set

common/sfc_efx/base: support adding encap action to a set

For convenience, there are two separate APIs provided, one for
adding the action and one for setting the encap. header ID.
This design allows the client driver to first build the action
set specification (which validates the order of the actions)
and, if everything is correct, proceed with allocation of the
resource utilised by the action set (encap. header). This
facilitates clarity of the client code and its efficiency.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>