* [swift5] Add useClasses to use `final class` instead of `struct`
* [swift5] Always include CodingKeys enum
* [swift5] Implement model equals and hash functions
* [swift5] Encode `null` values
* [swift5] Test `useClasses` in urlsessionLibrary
* [swift5] Add a required nullable prop test case to 2_0/swift/petstore*.yaml
* [swift5] Update samples and docs
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