Handle environments/:id/terminal.ws

A GitLab environment may expose a terminal connection for out-of-band access.
Workhorse is responsible for providing a websocket connection to the terminal
if present.

It authenticates the user and retrieves connection details from GitLab using
the environments/:id/terminal.ws/authorize endpoint, and sets up a proxy to
the terminal provider, converting from the remote's subprotocol to a common
format.

Authentication is periodically re-done, and the connection will be broken if
it fails, or if the connection details change in any way.