Add nullable support to Swift4

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

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

This will replace the unwrapRequired option.

Tests passed locally:

2019-08-28 18:24:13.135 xcodebuild[39128:3957771] [MT] IDETestOperationsObserverDebug: (2DC50469-4B44-406B-9B42-2EE085D7192E) Finished requesting crash reports. Continuing with testing.
All tests
Test Suite SwaggerClientTests.xctest started
PetAPITests
    ✓ test1CreatePet (0.697 seconds)
    ✓ test2GetPet (0.505 seconds)
    ✓ test3DeletePet (0.497 seconds)
StoreAPITests
    ✓ test1PlaceOrder (0.723 seconds)
    ✓ test2GetOrder (0.479 seconds)
    ✓ test3DeleteOrder (0.483 seconds)
UserAPITests
    ✓ testLogin (0.450 seconds)
    ✓ testLogout (0.530 seconds)


Executed 8 tests, with 0 failures (0 unexpected) in 4.362 (4.366) seconds
2019-08-28 18:27:13.958 xcodebuild[39128:3957771] [MT] IDETestOperationsObserverDebug: 205.584 elapsed -- Testing started completed.
2019-08-28 18:27:13.958 xcodebuild[39128:3957771] [MT] IDETestOperationsObserverDebug: 0.000 sec, +0.000 sec -- start
2019-08-28 18:27:13.958 xcodebuild[39128:3957771] [MT] IDETestOperationsObserverDebug: 205.584 sec, +205.584 sec -- end
▸ Test Succeeded
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 04:55 min
[INFO] Finished at: 2019-08-28T18:27:20+08:00
[INFO] ------------------------------------------------------------------------

cc @jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @d-date (2018/03)