Add "datetime" column data type to 2021-06-01-preview (#15682)

Author: yangmsft

specification/resourcegraph/resource-manager/Microsoft.ResourceGraph/preview/2021-06-01-preview/resourcegraph.json

@@ -394,7 +394,8 @@
         "integer",
         "number",
         "boolean",
-        "object"
+        "object",
+        "datetime"
       ],
       "x-ms-enum": {
         "name": "ColumnDataType",