Runtime configuration check

### Is your feature request related to a problem? Please describe.

In order to verify that a configuration is syntactically valid I can already run fluentd with the `--dry-run` flag. I would like to check if fluentd can actually start with the configuration.
This would help in cases when for example one specific output is not compatible with the available plugins, which we cannot detect using `--dry-run`.

### Describe the solution you'd like

I would like to be able to start fluentd with a configuration and tell it using some command line flags to start but quit with a zero exit code, if everything has been initialized properly.

### Describe alternatives you've considered

Right now I can achieve something similar by running fluentd using the `timeout` command, however that is suboptimal:
https://github.com/kube-logging/logging-operator/pull/1407

### Additional context

This feature would be leveraged by the logging operator in it's automatic config check feature:
https://github.com/kube-logging/logging-operator/issues/1432