Added support for getter sections in unions codegen

[timocov]

What behavior changes?

This PR adds GetterSection to getter of union variant classes.

Why is this change needed?

One of the places where GetterSection helps is when you want to add custom annotations to getters. For instance, when integrating generated code with jakarta validation in Spring Boot, complex fields require to have @Valid annotation attached in order to be validated (e.g. to run full depth validation). Currently, GetterSection is not "emitted" which means that union member getters aren't being processed and it is possible to add @Valid annotation in this case.

How was this validated?

I changed to code locally, run gw pTML, run the codegen, spin up a spring boot application and validated that the request is being validated correctly.

What should reviewers focus on?

Should I use

writer.pushState();
writer.injectSection(new GetterSection(member));

instead to avoid people accidentally removing the content of the method?

Also, should I do the same for enums?

Additional Links

N/A.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[adwsingh]

I wonder if the better design here is to generically be able to add support for custom Annotations in the codegen.