Support setting child errors in custom validators without using validateNested

[SharkSharp]

Problem

In class-validator, custom validators cannot interact with children errors directly.
The only mechanism that seems to support nested errors is @ValidateNested(), which is not always appropriate for custom validation scenarios.
This makes it hard to represent structured validation failures when a custom constraint needs to validate complex child state.

Motivation

I was considering forking the project and implementing this myself, but wanted to check first if there are any existing plans or preferred approaches.

Proposed Solution

My first thought is to extend defaultMessage (or add a similar method) so it can return a ValidationError structure directly.

Conclusion

I'm willing to implement the new feature, so if a direction is agreed on, I can build it and submit a PR.