[java][webclient] 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 webclient 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][feign] Fix model combining properties and additional properties #19713 for the feign library.

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[wing328]

@JoaoBrlt thanks for the PRs.

Can you please PM me via Slack to further discuss your PRs?

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