fix(schema): add unique identifiers for UnderlayLink relationships

Infrahub requires unique identifiers when multiple relationships
point to the same peer type with the same direction.

Ref: #41

schemas/extensions.yml

@@ -85,15 +85,19 @@ nodes:
         kind: Parent
       - name: local_device
         peer: InfraDevice
+        identifier: underlaylink__local_device
         cardinality: one
       - name: local_interface
         peer: InfraInterfaceEthernet
+        identifier: underlaylink__local_interface
         cardinality: one
       - name: remote_device
         peer: InfraDevice
+        identifier: underlaylink__remote_device
         cardinality: one
       - name: remote_interface
         peer: InfraInterfaceEthernet
+        identifier: underlaylink__remote_interface
         cardinality: one
 
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