[java][restclient] 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 #20853 (restclient library).

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[martin-mfg]

Hi @JoaoBrlt, thanks for the PR! The problem described in #20795 (for resttemplate) and fixed in #20902 also applies here (for restclient). Could you please fix it?

In fact the same problem also applies to webclient and feign. If you could fix it there too, that would be fantastic. :)

[JoaoBrlt]

Hi @martin-mfg! 👋 Oh sorry, haven't had time to check my notifications until now. That's a very good point. I copied the Mustache template code for the hashCode and equals methods from the native library which was one of the existing libraries properly supporting additionalProperties with Jackson. Indeed, they should be fixed to account for situations where there are only additional properties. It's cool that there's already a PR to fix that. 👌