Is your feature request related to a problem? Please describe.

When defining a RuleSet along with properties to be validated (like described here) the ValidationStrategy builds a CompositeValidatorSelector which then uses Any to combine IValidatorSelectors.

There are cases where it would be feasible to change Any to All since a RuleSet might define other rules for a property or there is no need to run validators not defined in a RuleSet using an explicit property.
Currently that is not possible.

Describe the solution you'd like

Something like this:

public enum ValidatorSelectorCombineMode
{
    All,
    Any,
}

When set to All the above described case will be possible.
Any is the currently default method to keep backward compatibility.

Describe alternatives you've considered

I'm willing to make a PR if this addition is welcome.

Additional Context

A sample which describes when to use this feature:

internal class ClientDetailsModelValidator : AbstractValidator<Models.ClientDetails>
{
    public ClientDetailsModelValidator()
    {
        RuleSet("name", () =>
        {
            RuleFor(m => m.Surname)
                .NotEmpty()
                .MaximumLength(50)
            ;

            RuleFor(m => m.Name)
                .NotEmpty()
                .MaximumLength(50)
            ;

            RuleFor(m => m.Patronymic)
                .MaximumLength(50)
            ;
        });

        RuleFor(m => m.Sex)
            .NotEmpty()
        ;

        RuleFor(m => m.Birthday)
            .NotEmpty()
            .LessThanOrEqualTo(DateTime.Today)
            .ExclusiveBetween(new DateTime(1900, 01, 01), new DateTime(2100, 01, 01))
        ;
    }
}

Currently it's not possible to validate a Surname property from name RuleSet separately.
Or at least I haven't found a way for it to work in a single Validate call.