Add featureId to auth_userState (#880)

This change adds an optional field to auth_userState that indicates
which feature is being referred to. This change supports an upcoming
change to the VS Toolkit.

## License

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Author: awschristou

telemetry/definitions/commonDefinitions.json

@@ -2701,6 +2701,10 @@
                     "type": "credentialType",
                     "required": false
                 },
+                {
+                    "type": "featureId",
+                    "required": false
+                },
                 {
                     "type": "source",
                     "required": true