[REQ] Supporting guava's Immutable Collections in Java codegen

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

Using immutable data structures is quite valuable when writing multi-threaded code, and I think it would be a great improvement to the current generated code if it supported generating immutable models and APIs.

Describe the solution you'd like

The end goal would be to add a CLI option to generate models/APIs which use guava's Immutable types, have no setters, and include generated builders. The first step I see is to initially add support for using guava's immutable collections, and future iterations may update templates to optionally remove getters and generate builders.

Describe alternatives you've considered

The only alternative I can think of is manually writing immutable wrappers for each generated model (per-project), which essentially defeats the purpose of generating code. At my workplace we used a similar solution with manually writing immutable wrappers, and we have since transitioned to generating immutable models.

Additional context

I've written up a proof-of-concept PR which adds support for generating code with guava's immutable collections, and wrote up more information about the implementation and hardships there.

https://github.com/OpenAPITools/openapi-generator/pull/5826

I'm curious to learn what the community thinks about improving support for immutable generated code.

Start by reviewing the proof-of-concept pull request #5826 and the proposed first step in this issue. The initial scope is support for Guava immutable collections in generated Java models and APIs; the broader CLI option, builder generation, and setter changes are described as future iterations.