add option to enable ssh in server options

[Panaetius]

We should add a boolean to enable SSH in server options when launching a session. This in turn sets an env variable RENKU_ENABLE_SSH in the session (which in turn has to be picked up here, optionally starting sshd).

the /servers GET request should return that information.

A deployment has to support SSH (via appropriate ingress/port setting) for SSH to be supported.