[swift5] Make it possible to opt out of JSONEncodable conformance

[stjernegard]

Fixes #12130

JSONEncodable is not a native type and does not exist if only models are generated. We use OpenAPI to generate only the models and none of the utils or other parts, so we need the generated models to be valid and able to compile as a stand-alone package.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

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[4brunu]

@stjernegard Hey, thanks for creating this PR.
Looks good to me.
Let's wait for CI to finish.
Thanks

[stjernegard]

Thanks :)
I noticed that my editor had chosen the wrong indentation level, so I’ve fixed that to match the rest of the file.

[4brunu]

LGTM