feature: Support HTTP request context in custom decorators

[tropical-coder]

It’d be great if class-validator could inject the original HTTP request (e.g., req.subdomains) into custom validator constraints—without hacks like manipulating the DTO or using interceptors.

---

Motivation – Subdomain-based validation:

export class MyDto {
  @IsValidForSubdomain()
  someProp: string;
}
// Custom validator should access req.subdomains directly.

---

Current workaround (ugly):

1. Interceptor attaches req.subdomains to DTO.
2. Validator reads it via args.object['subdomains'].

Messy, collision-prone, and framework-specific.

---

Proposed API:

export function IsValidForSubdomain(
  validationOptions?: ValidationOptions
) {
  return function (object: Object, propertyName: string) {
    registerDecorator({
      name: "isValidForSubdomain",
      target: object.constructor,
      propertyName: propertyName,
      options: validationOptions,
      validator: {
        validate(value: any, args: ValidationArguments, req) {
          return req.subdomains.includes('api') ? customCheck(value) : true;
        }
      }
    });
  };
}

Benefits:

• Clean and explicit
• No DTO pollution
• Framework-agnostic and safer DX