KSENROLL-14743 Add support for environment passthrough in api change detection

Additional environment variables like the pull request commit id may be
available in the environment.

We don't have to use them here but they are valuable for the downstream jobs
created from the generated files.

So add a mechanism where the plugin can be configured with the names of
environment variables which if they exist should be included in the generated
files.
KSENROLL-14743 Refactor some common parameters up into the base classes

Move some fields up into the base classes and then mark the setter's as final
so that the setters in the concrete classes can be found and removed.