feature: ValidateIfElse decorator for conditional custom validation with selective validator ignoring

[itamar-gerasy]

Description

Currently, @ValidateIf allows ignoring all validators on a property based on a condition, but there's no built-in way to conditionally apply custom validation logic while selectively ignoring other validators.

Developers often need a pattern where:

• If condition A is true → apply custom validation logic AND ignore all other validators
• If condition A is false → skip custom validation AND allow other validators to run

Current workarounds are problematic:

1. Using @ValidateIf inside custom validators ignores the custom validator itself
2. Manually manipulating validation metadata leads to complex, error-prone code
3. No clean way to achieve "either custom validation OR standard validators, but not both"

Use case example:

class User {
  @ValidateIfElse(
    (obj) => obj.isAdmin,           // condition
    (value) => value.length > 10,   // custom validation when admin
    { message: 'Admin names must be longer than 10 chars' }
  )
  @IsNotEmpty()  // ignored when isAdmin=true, applied when isAdmin=false
  @MinLength(3)  // ignored when isAdmin=true, applied when isAdmin=false
  name: string;
  
  isAdmin: boolean;
}

[mickosi]

I HAVE THE PROBLEM and need something like all AI recommend but not work

  @ValidateIf((obj) => first some condition)
  @IsNotEmpty()  // ignored when isAdmin=true, applied when isAdmin=false // apply that only if  first some condition is true
  @MinLength(3)  // ignored when isAdmin=true, applied when isAdmin=false // apply that only if  first some condition is true
// apply first group of decorators or second
  @ValidateIf((obj) =>  second some condition)  // or @ValidateElse()
  @IsNotEmpty()  // ignored when isAdmin=true, applied when isAdmin=false // apply that only if  second some condition is true
  @MinLength(3)  // ignored when isAdmin=true, applied when isAdmin=false // apply that only if  second some condition is true
  name: string;
```/