encoding/gob: add RegisteredNames()

Starting from Go 1.23, direct access to `encoding/gob.nameToConcreteType` is no longer possible due to the removal of internal symbol linking. While this change improves encapsulation, it also removes the ability to retrieve a list of all registered types in the gob package.

Relates: #67401

Closes: #71602

Author: laskoviymishka

Diff for: src/encoding/gob/type.go

@@ -899,6 +899,17 @@ func Register(value any) {
 	RegisterName(name, value)
 }
 
+// RegisteredNames is list of registered type names that registered
+// with [Register] or [RegisterName] methods
+func RegisteredNames() []string {
+	var val []string
+	nameToConcreteType.Range(func(k, v any) bool {
+		val = append(val, k.(string))
+		return true
+	})
+	return val
+}
+
 func registerBasics() {
 	Register(int(0))
 	Register(int8(0))

Diff for: src/encoding/gob/type_test.go

@@ -260,3 +260,10 @@ func TestTypeRace(t *testing.T) {
 	close(c)
 	wg.Wait()
 }
+
+func TestRegisteredNames(t *testing.T) {
+	names := RegisteredNames()
+	if len(names) < 34 {
+		t.Errorf("registered names should contains all primitive type")
+	}
+}