add x-ms-enum for orbital (Azure#17238)

Author: ArcturusZhang

specification/orbital/resource-manager/Microsoft.Orbital/preview/2021-04-04-preview/orbital.json

@@ -1785,6 +1785,10 @@
         "EarthObservation",
         "Communication"
       ],
+      "x-ms-enum": {
+        "name": "CapabilityType",
+        "modelAsString": true
+      },
       "default": "EarthObservation",
       "description": "Ground Station Capability",
       "x-ms-parameter-location": "method"