Add userId field to pubsub protocol messages

### Is your feature request related to a problem? Please describe.
When using **json.webpubsub.azure.v1sub** protocol, and `sendToGroup` the clients that receive the messages, have no way of getting or seeing the user that sent the message. This field is not in the payload anywhere there seems to be no way to access it.

The only solution is to use server side event handlers to intercept the message, add the userId field in a custom property, e.g. `from:` and send to clients. This seems like a big overhead for such a standard feature

### Describe the solution you'd like
Add userId field to the payload send to clients when `sendToGroup` messages are used.

### Additional context
None

No files, tests, or implementation entry points are identified in the issue. Start by reviewing the json.webpubsub.azure.v1sub protocol and the sendToGroup message payload; done means clients receiving those messages can access the sender's userId without a server-side event handler.

Written by the indexing model from the issue text.