Use RuntimeException instead of checked exceptions (ApiException)

[dominik-kovacs]

according to docs it's possible to generate the client without checked exceptions
https://openapi-generator.tech/docs/generators/java/
parameter useRuntimeException

It would reduce lots of boilerplate code, since these exception are usually converted to unchecked exceptions, not bringing actual value

[jmini]

Thank you for this feedback.

We are currently investigating the impact of such a change.