feat: Add WithNameFunc to Validator (#28)

## Motivation

It's often the case that we wan't to dynamically construct the validated
entity's name.
It would be useful to provide a function that can do that for us.

## Release Notes

Added `Validator.WithNameFunc` function which can be used to dynamically
set the validator's name per-instance.

pkg/govy/example_test.go

@@ -72,6 +72,25 @@ func ExampleValidator_WithName() {
 	//   - always fails
 }
 
+// If statically defined name through [govy.Validator.WithName] is not enough,
+// you can use [govy.Validator.WithNameFunc].
+// The function receives the entity's instance you're validating and returns a string name.
+func ExampleValidator_WithNameFunc() {
+	v := govy.New(
+		govy.For(func(t Teacher) string { return t.Name }).
+			Rules(govy.NewRule(func(name string) error { return fmt.Errorf("always fails") })),
+	).WithNameFunc(func(t Teacher) string { return "Teacher " + t.Name })
+
+	err := v.Validate(Teacher{Name: "John"})
+	if err != nil {
+		fmt.Println(err)
+	}
+
+	// Output:
+	// Validation for Teacher John has failed for the following properties:
+	//   - always fails
+}
+
 // You can also add [govy.Validator] name during runtime,
 // by calling [govy.ValidatorError.WithName] function on the returned error.
 //

pkg/govy/validator.go

@@ -20,18 +20,28 @@ func New[S any](props ...validationInterface[S]) Validator[S] {
 // It serves as an aggregator for [PropertyRules].
 // Typically, it represents a struct.
 type Validator[S any] struct {
-	props []validationInterface[S]
-	name  string
+	props    []validationInterface[S]
+	name     string
+	nameFunc func(S) string
 
 	predicateMatcher[S]
 }
 
 // WithName when a rule fails will pass the provided name to [ValidatorError.WithName].
 func (v Validator[S]) WithName(name string) Validator[S] {
+	v.nameFunc = nil
 	v.name = name
 	return v
 }
 
+// WithNameFunc when a rule fails extracts name from provided function and passes it to [ValidatorError.WithName].
+// The function receives validated entity's instance as an argument.
+func (v Validator[S]) WithNameFunc(f func(s S) string) Validator[S] {
+	v.name = ""
+	v.nameFunc = f
+	return v
+}
+
 // When accepts predicates which will be evaluated BEFORE [Validator] validates ANY rules.
 func (v Validator[S]) When(predicate Predicate[S], opts ...WhenOptions) Validator[S] {
 	v.predicateMatcher = v.when(predicate, opts...)
@@ -75,7 +85,11 @@ func (v Validator[S]) Validate(st S) error {
 		allErrors = append(allErrors, pErrs...)
 	}
 	if len(allErrors) != 0 {
-		return NewValidatorError(allErrors).WithName(v.name)
+		name := v.name
+		if v.nameFunc != nil {
+			name = v.nameFunc(st)
+		}
+		return NewValidatorError(allErrors).WithName(name)
 	}
 	return nil
 }

pkg/govy/validator_test.go

@@ -97,6 +97,20 @@ func TestValidatorWithName(t *testing.T) {
     - test`)
 }
 
+func TestValidatorWithNameFunc(t *testing.T) {
+	r := govy.New(
+		govy.For(func(m mockValidatorStruct) string { return "test" }).
+			WithName("test").
+			Rules(govy.NewRule(func(v string) error { return errors.New("test") })),
+	).WithNameFunc(func(m mockValidatorStruct) string { return "validator with field: " + m.Field })
+
+	err := r.Validate(mockValidatorStruct{Field: "FIELD"})
+	assert.Require(t, assert.Error(t, err))
+	assert.EqualError(t, err, `Validation for validator with field: FIELD has failed for the following properties:
+  - 'test' with value 'test':
+    - test`)
+}
+
 func TestValidatorInferName(t *testing.T) {
 	r := govy.New(
 		govy.For(func(m mockValidatorStruct) string { return "test" }).