[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 https://github.com/OpenAPITools/openapi-generator/issues/17361 only for the feign library.

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

• https://github.com/OpenAPITools/openapi-generator/pull/19706 for the resttemplate library.
• https://github.com/OpenAPITools/openapi-generator/pull/19711 for the webclient library.

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