Validation: conditional validators

[System-Glitch]

Proposal

Sometimes we want a validator to be executed only if a certain condition is matched. For example, we want a validator on a field only if another field has a certain value.

It is currently possible but inconvenient to do so. We should introduce a new OnlyIf validator (or another more suited name if you have better ideas) that only executes a given validator if a condition is met, otherwise returns true.

Example usage:

v.OnlyIf(func(ctx *v.Context) bool {
	other := walk.MustParse("other").First(ctx.Data)
	return other.Value == true
}, MyValidator())

In this example, MyValidator would only be executed if the other field has the value true. If the condition returns false, the validation passes without executing MyValidator.

The message placeholders and other properties must be inherited from MyValidator.