fix(schema): use direction outbound for UnderlayLink relationships

Use outbound direction to avoid bidirectional relationship conflicts
when multiple relationships point to the same peer type.

Ref: #41

schemas/extensions.yml

@@ -85,20 +85,24 @@ nodes:
         kind: Parent
       - name: local_device
         peer: InfraDevice
-        identifier: underlaylink__local_device
+        identifier: underlay_link_local_device
         cardinality: one
+        direction: outbound
       - name: local_interface
         peer: InfraInterfaceEthernet
-        identifier: underlaylink__local_interface
+        identifier: underlay_link_local_interface
         cardinality: one
+        direction: outbound
       - name: remote_device
         peer: InfraDevice
-        identifier: underlaylink__remote_device
+        identifier: underlay_link_remote_device
         cardinality: one
+        direction: outbound
       - name: remote_interface
         peer: InfraInterfaceEthernet
-        identifier: underlaylink__remote_interface
+        identifier: underlay_link_remote_interface
         cardinality: one
+        direction: outbound
 
   # =============================================================================
   # Host Connection