Input validation for application endpoint

[ztefanie]

Is your feature request related to a problem? Please describe.
Currently sending bad requests to the application endpoint and results in 500 server errors. This should not be the case and proper error handling should be added and proper error messages should be sent.

Describe the solution you'd like
Check all values of the application endpoint. What happens if they are set to "wrong" values. E.g. if setting `"givenInformationIsCorrectAndComplete" to false a 500 error is thrown or if an empty string is sent for any value (like forename) the server sends a 500 error.
Also if sending multiple query / muntations at once leads to an 500 error

Additional context
As this endpoint is open it should handle all kinds of requests that it gets properly.

[ztefanie]

I already moved this to next up, as I think when verein360 starts to use this endpoint, it would make sense we soonish validate input properly