Move session runners to own service

Currently, the session runner (with the Kubernetes operator) is part of the backend.

We shall move the session handling out of the backend and into separate session runners. A session runner would be comparable to Gitlab runners.

There shall be one default runner, which is auto-deployed in the Helm chart. The runner will be registered as default runner, so that there is no change in behaviour.

Also, custom runners can be installed on different machines, e.g. a virtual machine. This can be useful for power-users with high performance and low latency requirements, where Guacamole and the session have to run on one node. A setup with the session and Guacamole on the same node can be accomplished together with this PR: https://github.com/DSD-DBS/capella-collab-manager/issues/1114

There has to be runner management UI in the frontend. Also, specific runners should be assignable according to specific criteria to specific sessions. For the beginning, we should be able to select a specific runner per user.