Proxyd should have a configuration check option

[optimisticben]

Is your feature request related to a problem? Please describe. Proxyd should have on option to verify a config is valid without having to start the server process.

Describe the solution you'd like Something like proxyd checkconfig --config <PATH-TO-FILE> Proxyd should use exit code 1 if the configuration is invalid.

Additional context If proxyd is restarted with an invalid configuration the service will be down until the situation is fixed.