feat: implement ValidatorValuer interface feature

[thommeo]

Problem

In order to have custom type validations for generic types, every possible type has to be registered with RegisterCustomTypeFunc to return the underlying type. It is especially relevant for nullable types e.g. sql.Null[T].

Discussed here: #1232

Fixes Or Enhances

• Adds a mechanism for validating generic types without using RegisterCustomTypeFunc by introducing ValidatorValuer interface.

Example:

type Nullable[T any] struct {
	Data T
}

func (t Nullable[T]) ValidatorValue() any {
	return t.Data
}

type Config struct {
	Name string `validate:"required"`
}

type Record struct {
	Config Nullable[Config] `validate:"required"`
}

func TestValidatorValuerInterface2(t *testing.T) {
	s := Record{
		Config: Nullable[Config]{},
	}
	v := New()
	errs := v.Struct(s)
	t.Fatalf("Error: %v", errs.Error())
}

// Key: 'Record.Config.Name' Error:Field validation for 'Name' failed on the 'required' tag
// Notice: no Data in the Record.Config.Name

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 73.659% (+0.006%) from 73.653%
when pulling a4b2773 on thommeo:feat/validation-valuer
into c3fc72e on go-playground:master.