[swift5][client] add option to generate or not the model additional properties

[4brunu]

Fixes #8895

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
  

  Commit all changed files.
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@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)

[wing328]

As discussed, the codegen (default) should provide a way to loop through all models and create an extension to indicate whether there's any models with additional properties. We'll try to add that to default codegen in the future.

For the time being, this PR looks good.