[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.

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@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.

[wing328]

@JoaoBrlt when you've time this week, can you please PM me via Slack to go over your PRs and get the merged?

[wing328]

@JoaoBrlt thanks again for the PR.

When you've time, can you please resolve the merge conflicts and fix the CI failure?

[JoaoBrlt]

Hi @wing328 ! 👋 No problem, I've just rebased the branch to fix the merge conflicts and the CI failure.