feat: Add AllowIf Decorator

[burriedu2]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

There is currently no way to define whether a property is conditionally whitelisted, for example:

enum TypesCancel = {
  USER: 'user',
  STOCKOUT: 'stockout',
  OTHERS: 'others',
}

class CancelDTO {
  @IsEnum(TypesCancel)
  @IsNotEmpty() 
  type: TypesCancel;

  // if the type === TypesCancel.OTHERS, reason is required, but if not, must not sent
  @Optional() // with optional, it still receives even if the type !== TypesCancel.OTHERS
  @IsString()
  reason?: string;
}

const cancel = new CancelDTO();
cancel.type = TypesCancel.USER;
cancel.reason = 'reason';

validate(cancel).then(errors => {
  // cancel.reason is defined
});

Issue Number: typestack#1489

What is the new behavior?

enum TypesCancel = {
  USER: 'user',
  STOCKOUT: 'stockout',
  OTHERS: 'others',
}

class CancelDTO {
  @IsEnum(TypesCancel)
  @IsNotEmpty() 
  type: TypesCancel;

  @AllowIf(cancel => cancel.type === TypesCancel.OTHERS) // with AllowIf, the reason was accept if type === TypesCancel.OTHERS
  @IsString()
  reason?: string;
}

const cancel = new CancelDTO();
cancel.type = TypesCancel.USER;
cancel.reason = 'reason';

validate(cancel).then(errors => {
  // errors = reason should not exist
  // cancel.reason is not defined
});

Does this PR introduce a breaking change?

• Yes
• No

Other information

This PR contains all changes from #86 but it has been rebased since the master has changed since it was submitted. It also updates the behavior to work as expected if forbidNonWhitelisted is set.