Allow using tags in StructFiltered

[masterada]

Package version eg. v8, v9:

v10

Issue, Question or Enhancement:

It would be nice if there was a version of StructFiltered, where we can use tags instead of field values. It would be best to pass in a type similar to FieldError, instead of ns []byte.

Code sample, to showcase or reproduce:

func TestTest(t *testing.T) {
	v := validator.New()

	v.RegisterTagNameFunc(func(field reflect.StructField) string {
		name := strings.SplitN(field.Tag.Get("json"), ",", 2)[0]
		if name != "" && name != "-" {
			return name
		}

		return ""
	})

	type TestStruct struct {
		TestedField    string `json:"tested_field" validate:"required"`
		NotTestedField string `json:"not_tested_field" validate:"required"`
	}

	testVal := &TestStruct{
		TestedField:    "test",
		NotTestedField: "",
	}

	err := v.StructFiltered2(testVal, func(meta validator.FieldMeta) bool {
		return meta.Field() == "not_tested_field"
	})
	if err != nil {
		t.Error(err)
	}
}

Background

We use validator for a rest api. We allow partial updates to our entities. We do this by unmarshalling the incoming data to a struct, and also to a map[string]interface{}. This way we have the sent-in keys in the map, and use it to apply the incoming changes to our existing data.