codegen-java: Support generating record classes

codegen-java should support generating record classes instead of regular classes whenever possible.
(codegen-kotlin already generates data classes whenever possible.)

I have this almost working, but one important question remains: how to deal with backward compatibility?

One option is to introduce a `—generate-records` flag and only generate record classes if this flag is set.
Another option is to ask users to stay on the 0.27 code generator until they are ready to switch to record classes.
In any case, generating record classes should eventually become the default.

Start in the codegen-java implementation and compare its generation behavior with codegen-kotlin, which already generates data classes when possible. Review the backward-compatibility question around a —generate-records flag versus changing the default. Done means record classes are supported whenever possible and the compatibility and default behavior are decided.

Written by the indexing model from the issue text.