[java][feign] Fix model combining properties and additional properties

[JoaoBrlt]

When additionalProperties is set to true, use an additionalProperties attribute and @JsonAnySetter and @JsonAnyGetter annotations instead of extending HashMap.

This fixes the issue #17361 only for the feign library.

I also created the following pull requests to fix the same issue for other libraries:

• [java][resttemplate] Fix model combining properties and additional properties #19706 for the resttemplate library.
• [java][webclient] Fix model combining properties and additional properties #19711 for the webclient library.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08) @wing328

[lesteenman]

What's needed to get this merged?

[martin-mfg]

What's needed to get this merged?

Hi, same as in this comment - @wing328 needs to check and merge this PR.