[REQ] [java] Fluent client method builder

Is your feature request related to a problem? Please describe.

N/A

Describe the solution you'd like

I'd like to have an option to allow generation of fluent builders for each operation.

This might tie well with #18032.

Describe alternatives you've considered

I can emulate this with a custom template that adds Lombok's Builder on instance methods, with builder class name and build method name that reference operatioId and the 2 variant suffixes. Change build method to something like invoke and you have something that works.

Additional context

Making this a feature here instead of relying on Lombok means we could generate staged builders for handling required parameters.

I'd like this backported to 6.x.

If you find it worthwhile, on a more general purpose level, some kind of parameter object + staged builder combo annotation processor might be more useful for the community at large.

Start by reading issue #18032 and reviewing how the existing custom-template approach generates Java operation methods. Define the scope for fluent and staged builders, including required parameters, operationId-based names, variant suffixes, and the requested 6.x backport; completion should include an agreed design and coverage for generated clients.