New checks

[tcompa]

1. A check based on pg_isready (https://www.postgresql.org/docs/current/app-pg-isready.html), which takes the database name as an argument.
2. A check that parses recent logs of e.g. fractal-server (journalctl -u fractal-server --since today) and looks for "error" or "warning" records
3. A check that a given server is SSH-reachable (this can be implemented through paramiko, or directly by running something like ssh user@host whoami
4. A check that the vizarr service is alive (ref Introduce an alive endpoint fractal-vizarr-viewer#54)
5. An improved version of the current json_url check, which also requires the JSON to contain e.g. the alive=true property.
6. A check on HTTPS-certificates validity