Add support of useOneOfInterfaces for Spring and all JaxRS based generators

[rfecher]

This is actually referenced here and it addresses issue #5381 nicely.

I'm willing to do the legwork to get the PR in, however I'm not actually the author, and I'd think to maintain the license agreement we'd need consent from @mmalygin

PR checklist

• [ X ] Read the contribution guidelines.
• [ X ] If contributing template-only or documentation-only changes which will change sample output, build the project before.

this step is unclear to me, I run a bunch of the scripts and it seems many seemingly unrelated files changed like all the FakeApi.java files changed put("http", EncodingUtils.encodeCollection(value, "ssv")); to put("http", EncodingUtils.encodeCollection(value, "space")); which seems completely outside of the scope of this bugfix

• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• [ X ] File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
  further guidance would be appreciated here
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[wing328]

@rfecher thanks for the PR. Can you please PM me via Slack to discuss this PR?

https://join.slack.com/t/openapi-generator/shared_invite/enQtNzAyNDMyOTU0OTE1LTY5ZDBiNDI5NzI5ZjQ1Y2E5OWVjMjZkYzY1ZGM2MWQ4YWFjMzcyNDY5MGI4NjQxNDBiMTlmZTc5NjY2ZTQ5MGM

[dang-gyg]

hey guys, any updates on this PR?

[GeertZondervan]

Good day @rfecher, @wing328 ,

Could we get an update on this PR?

Currently, none of the JAX-RS generators handle oneOf correctly. With #8091, the foundational work in the Java base (abstract) code generation classes has already been completed. I did a quick test and it looks like all what needs to be done is to integrate the JAX-RS Mustache templates from this PR to ensure that proper interfaces are created when useOneOfInterfaces is enabled.