[C][Client] Support free-form objects

Created by: ityuhui

@wing328 @zhemant @michelealbano

The PR https://github.com/OpenAPITools/openapi-generator/pull/12069 adds the support for free-form objects. That is a very useful feature. I'd like to submit the code on behalf of the author @harlequin-tech. So I picked up the two files and updated them according to my thoughts.

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