S3ng: S3 health /ready check

## Is your feature request related to a problem? Please describe.
To gain confidence of a up and running system I would love the storageusers service to do a S3 health check.

## Describe the solution you'd like
The storageusers service could do a regular HEAD on the configured bucket if S3ng is used. This would help gaining confidence, that the storageusers service has the right S3 credentials and is able to connect.

This does not need to be scheduled by oCIS iself - a command that I can invoke from the CLI would also be fine for me.

## Describe alternatives you've considered
Wait for a user to upload a file and see the upload failing

## Additional context
This would highly improve maintainability.