service: Support proxy forward auth using OAuth2 Proxy

roles/service/meta/argument_specs.yaml

@@ -33,6 +33,16 @@ argument_specs:
         type: bool
         required: false
         default: true
+      service_proxy_auth_type:
+        description: >-
+          Set to oauth2-proxy to use OAuth2 Proxy for vhost authentication.
+          The oauth2-proxy role must be run separately.
+        type: str
+        required: false
+        default: none
+        choices:
+          - none
+          - oauth2-proxy
       service_vhost_locations:
         description: Passed to vhost role as vhost_locations
         type: list