[server] Add analytics API

[svenefftinge]

In order to send events from the dashboard and other places (vs code, local companion app, etc.) to our analytics end-point, the server should expose an API. I think it would be easiest to add a trackEvent method to the websocket protocol.

[csweichel]

We already have the AnalyticsWriter API in gitpod-protocol. We could implement the AnalyticsWriter interface on the gitpod-server API. Downside is that we bloat extend that API further. Upside: we get authentication along for the ride and don't have to expose the segment write key/config.

[svenefftinge]

we get authentication along for the ride and don't have to expose the segment write key/config.

Yes, I think adding it as a message to gitpod-service is easy and preferable also because it centralizes the analytics strategy (and its config).