examples/ipsec-secgw: support IPv6 options

Using transport with IPv6 and header extensions requires calculating
total header length including extensions up to ESP header which is
achieved with iteratively parsing extensions when preparing traffic
for processing. Calculated l3_len is later used to determine SPI
field offset for an inbound traffic and to reconstruct L3 header by
librte_ipsec.

Signed-off-by: Marcin Smoczynski <marcinx.smoczynski@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>
Tested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>