examples/ipsec-secgw: add event worker launch

examples/ipsec-secgw: add event worker launch

In eventmode workers can be drafted differently according to the
capabilities of the underlying event device. The added functions will
receive an array of such workers and probe the eventmode properties to
choose the worker.

Signed-off-by: Anoob Joseph <anoobj@marvell.com>
Signed-off-by: Lukasz Bartosik <lbartosik@marvell.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>