* Document Kubernetes required permissions

* Clarify that ClusterRole is required

* Add Run.next_triggered_at

* Return next_triggered_at=None for finished runs

* Show Schedule and Next run on Run page

Add an offer field for holding arbitrary
backend-specific data. As an example, use this
field to mark reserved GCP A4 instances.

* Show Finished for runs and jobs in the UI

* Do not show Finished for pending runs

This commit updates the CLI and the server to use
server-managed user SSH keys when starting new
runs. This allows users to attach to the run from
different machines, since the SSH key is
automatically replicated to all clients.

Implementation details:
- Server:
    - If the user key is missing, generate it when
      the user first calls `/get_my_user`.
- Client:
    - Before applying or getting a run plan, call
      `/get_my_user` to check if the user key is
      available. If it is, use it.
    - Cache the downloaded keys in `~/.dstack/ssh`
      to avoid repeated `/get_my_user` calls.
    - Switch from `warn` to logger messages, since
      this code is part of the Python API, so its
      output should be configurable.

Co-authored-by: Jvst Me <[email protected]>