[swift5]Add validation rule and validator

[maoyama]

#8024 is same in Swift, so I implemented.
First of all, Strings and Numbers are supported. Others will be added in due course.

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[maoyama]

@4brunu May I please get a review, thank you 🙏

[4brunu]

Hi @maoyama, CI is failing, could you please check what's wrong? Thanks

[4brunu]

Looks good to me 👍

[honkmaster]

@4brunu @maoyama Our API returns DTOs named ValidationError. Thus, we get invalid redeclaration errors. Is it possible to make the name of structs introduced with this PR changeable?

[4brunu]

I think we can rename ValidationError, because it's a common error, to something like OpenAPIValidationError.
@honkmaster can you open a PR with this change?