[Swift 4] add nullable support

[4brunu]

This PR is a continuation of #3783

Add nullable (or x-nullable in OAS2) support to swift4

This will replace the unwrapRequired option.

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• Read the contribution guidelines.
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@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)

[4brunu]

@wing328 has talked in #4754, I think this now is ready to merge 🙂

[4brunu]

Close in favor of #4086