Update enum of billing timegrain for budget API

Author: REDMOND\chanwan

specification/consumption/resource-manager/Microsoft.Consumption/stable/2019-01-01/consumption.json

@@ -2093,7 +2093,10 @@
           "enum": [
             "Monthly",
             "Quarterly",
-            "Annually"
+            "Annually",
+            "BillingMonth",
+            "BillingQuarter",
+            "BillingAnnual"
           ],
           "x-ms-enum": {
             "name": "TimeGrainType",