Identify who sent what event

Another issue discussed at the Jupyter server meeting 29/4 https://github.com/jupyter-server/team-compass/issues/4#issuecomment-830405683.

Currently there's no way to identify/record who sends an event. This is a blocker for Jupyter Server client event via the `eventlog` endpoint since without this, anyone can send any event whose schema has been registered, *e.g.* a client would be able to emit a fake server event or mimic another client's events...

The JEP mentioned [authenticated user](https://github.com/jupyter/telemetry/blob/master/proposal/JEP.md#authenticated-routing-service) but only for JupyterHub.

One potential approach proposed by Min is JSON Web Token (JWT) where users sign the event using JWT.

Start with the linked JEP section on authenticated routing and the team meeting discussion, then inspect how the eventlog endpoint currently accepts events. Compare the authenticated-user and JWT approaches, and define a design that records the sender and prevents clients from spoofing server or other-client events.

Written by the indexing model from the issue text.