add stack_type attribute (GoogleCloudPlatform#9890)

mmv1/products/compute/InterconnectAttachment.yaml

@@ -300,3 +300,15 @@ properties:
       - :IPSEC
     default_value: :NONE
     custom_flatten: templates/terraform/custom_flatten/default_if_empty.erb
+  - !ruby/object:Api::Type::Enum
+    name: 'stackType'
+    description: |
+      The stack type for this interconnect attachment to identify whether the IPv6
+      feature is enabled or not. If not specified, IPV4_ONLY will be used.
+
+      This field can be both set at interconnect attachments creation and update
+      interconnect attachment operations.
+    values:
+      - :IPV4_IPV6
+      - :IPV4_ONLY
+    default_from_api: true