Session status API

[nadiamoe]

Currently, clients can create and remove browser sessions, but they are not offered a way of knowing the state of the session. This can be useful to know if a browser has crashed if the client sees the WS connection terminate abnormally.

This endpoint could return data like:

• Process state (running, exited)
• Exit code (if existed)
• Stdout/stderr (up to a certian limit, hopefully)

Crocochrome could store this data for the last N sessions.

For this to be reliable, we'd need #122 in place.

This can be a building block to help with https://github.com/grafana/sm-k6-runner/issues/574