Changes

s1ap_proxy: handle_ies(): also handle #'ProtocolExtensionField'

This patch prepares for a follow-up change adding E-RAB MODIFY REQ/RSP,
allowing to execute handle_ies() on a list of #'ProtocolExtensionField'.

* 9.1.3.3 E-RAB MODIFY REQUEST
** contains an E-RABToBeModifiedListBearerModReq IE
*** contains E-RABToBeModifiedItemBearerModReq IEs
**** each may optionally contain TransportInformation IE
                                 ^^^^^^^^^^^^^^^^^^^^^^^
                                 #'ProtocolExtensionField'

Change-Id: Ic1ed2c4662e82bd739c35f9179eaaec20c3cad98