Issue54 Fix class loader and Prefixed Config option issues

[davidradl]

Description

The connector did not work when using the sql-client t Flink 1.16. This is because Flink 1.16 introduced a new way to load classes, which meant the existing code failed for 2 reasons:

• the code was not using the correct classloader to load classes, after Flink 1.16
• the PrefixedConfigOption, extended ConfigOption. In the past this worked as the parent class constructor was protected and the child class was in the same package. But after the classloader change, the parent class was loaded using a different classloader so its protected content was not accessible.

This fix

• ensures the appropriate class loader is used.
• amends the implementation of PrefixedConfigOption to avoid accessing protected content, by using reflection.

Resolves issue 54

PR Checklist

• [x] Tests added
• [x] Changelog updated

[davidradl]

I installed the Checkstyle plugin - it has been complaining in the local build, but it is clean locally - if there is an import order issue - I can tweak the order.