[Swift4] Introduce XcodeGen

[d-date]

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.
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Description of the PR

For Carthage users, we have no means just generating only Cartfile, and we need pbxproj file.
To resolve this issue, we introduce XcodeGen to generate .pbxproj.

Of course you can override XcodeGen.mustache with your requirements with -t option.

This is optional only for who can run xcodegen generate in your local machine or CI/CD.
We provide project.yml for XcodeGen users, and you can install generated Client with Carthage easily.

[wing328]

@d-date thanks for the PR, which has been included in the 4.1.0 release: https://twitter.com/oas_generator/status/1160000504455319553