[ObjC] Add a `unpack` helper for GPBAny with extension support.

The message included could be proto2 syntax and thus have extensions.

PiperOrigin-RevId: 586991004

objectivec/GPBWellKnownTypes.h

@@ -201,6 +201,24 @@ typedef NS_ENUM(NSInteger, GPBWellKnownTypesErrorCode) {
  */
 - (nullable GPBMessage *)unpackMessageClass:(Class)messageClass error:(NSError **)errorPtr;
 
+/**
+ * Unpacks the serialized message as if it was an instance of the given class.
+ *
+ * @note When checking type_url, the base URL is not checked, only the fully
+ *       qualified name.
+ *
+ * @param messageClass The class to use to deserialize the contained message.
+ * @param extensionRegistry The extension registry to use to look up extensions.
+ * @param errorPtr     Pointer to an error that will be populated if something
+ *                     goes wrong.
+ *
+ * @return An instance of the given class populated with the contained data, or
+ *         nil on failure.
+ */
+- (nullable GPBMessage *)unpackMessageClass:(Class)messageClass
+                          extensionRegistry:(nullable id<GPBExtensionRegistry>)extensionRegistry
+                                      error:(NSError **)errorPtr;
+
 @end
 
 NS_ASSUME_NONNULL_END

objectivec/GPBWellKnownTypes.m

@@ -195,6 +195,12 @@ static NSString *ParseTypeFromURL(NSString *typeURLString) {
 }
 
 - (GPBMessage *)unpackMessageClass:(Class)messageClass error:(NSError **)errorPtr {
+  return [self unpackMessageClass:messageClass extensionRegistry:nil error:errorPtr];
+}
+
+- (nullable GPBMessage *)unpackMessageClass:(Class)messageClass
+                          extensionRegistry:(nullable id<GPBExtensionRegistry>)extensionRegistry
+                                      error:(NSError **)errorPtr {
   NSString *fullName = [messageClass descriptor].fullName;
   if (fullName.length == 0) {
     if (errorPtr) {
@@ -215,10 +221,7 @@ static NSString *ParseTypeFromURL(NSString *typeURLString) {
     return nil;
   }
 
-  // Any is proto3, which means no extensions, so this assumes anything put
-  // within an any also won't need extensions. A second helper could be added
-  // if needed.
-  return [messageClass parseFromData:self.value error:errorPtr];
+  return [messageClass parseFromData:self.value extensionRegistry:extensionRegistry error:errorPtr];
 }
 
 @end