[kotlin-client][jvm-spring-restclient] Extract data from PartConfig for multipart/form-data requests in ApiClient

[zakupower]

Should fix the following error that was appearing when generating a API for multipart/form-data requests with kotlin jvm-spring-restclient: Fixes #20597

Content type 'multipart/form-data' not supported for bodyType=java.util.Collections$SingletonMap

Technical comitee: @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier

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[4brunu]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.
Let me know if you need help fixing it.
Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[zakupower]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors. Let me know if you need help fixing it. Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

Thanks, I didn't even notice I was using another account. I think I managed to change the author.