Removes final modifier from generated Proxy classes.

[cccs-ian]

Description

This PR removes the final modifier from the generated proxy classes, allowing clients code to extend them. This PR relates to some changes made here: https://github.com/brettwooldridge/HikariCP/pull/1661 where a small bug was fixed associated with the codegen.

Reasoning

My team has extended the HikariProxyResultSet in older versions of HikariCP, to allow us to package up stuff like warnings into the ResultSet implementation. I would be open to the suggestion that only the HikariProxyResultSet be made non-final, and be willing to change the PR to match if is decided that this is the best course of action.

Thanks for your consideration.

[cccs-ian]

Hoping to start conversation about this PR :)