Missing healthcheck probe to check if minio is working or just Pod is UP

[Barteus]

We have no indication if the minio application is working or not. It's important because juju thinks that Minio works fine when it is showing errors like - unable to connect (when using gateway mode with the wrong private endpoint). Currently, some errors flip the switch on juju to error but some are unnoticed.

We can use health check probes from minio documentation: https://docs.min.io/docs/minio-monitoring-guide.html