IHandoverPacketHolder: add eNB-specific handover gate to interface

HandoverPacketHolderEnb already has an hoManagerOut gate (used to
forward packets to the X2 forwarder during handover), but the gate
was declared only on the concrete eNB module, not on the
IHandoverPacketHolder module interface. Add it to the interface (and,
as @Loose, to HandoverPacketHolderUe) so that NED code which accesses
the submodule through the interface type can reference the gate, and
so custom holder implementations can be substituted.

Author: MohamedSeliem

src/simu5g/stack/ip2nic/HandoverPacketHolderUe.ned

@@ -25,4 +25,5 @@ simple HandoverPacketHolderUe like IHandoverPacketHolder
         input upperLayerIn;
         input x2In @loose; // required by the interface, but UNUSED
         output stackOut;
+        output hoManagerOut @loose; // required by the interface, but UNUSED
 }

src/simu5g/stack/ip2nic/IHandoverPacketHolder.ned

@@ -21,4 +21,5 @@ moduleinterface IHandoverPacketHolder
         input upperLayerIn;
         input x2In @loose; // only used in eNodeB/gNodeB nodes
         output stackOut;
+        output hoManagerOut @loose; // eNodeB/gNodeB only: to forward packets to handover manager for X2 tunneling
 }