feat(ubridge): probe iol_bridge's TAP-port support

Whether an IOU port can terminate on a persistent TAP (iol_bridge
add_nio_tap) is a capability of its own: the tap module and that command
land independently, so a build may have one without the other. Probed on
a scratch IOL bridge with a scratch TAP — one add/delete cycle, no
iol_bridge start needed (attaching a NIO works on a stopped bridge) —
cached by binary identity, with the scratch bridge id above the per-node
id space the fabric locks (513..1024) so a probe cannot collide with a
running node. Any failure, including an old build's "Unknown command",
means unknown: IOU stays on the relay datapath.

Reported through /capabilities as ubridge_iol_tap, next to ubridge_tap.

gns3server/controller/compute.py

@@ -101,6 +101,7 @@ class Compute:
             "node_types": [],
             "ubridge_tc": None,
             "ubridge_tap": None,
+            "ubridge_iol_tap": None,
         }
         self.name = name
         # Cache of interfaces on remote host