[ObjC] Add `-[GPBMessage clearUnknownFields]`.

This is in preparation for revising the Unknown Fields related apis.

PiperOrigin-RevId: 647693027

objectivec/GPBMessage.h

@@ -493,6 +493,15 @@ CF_EXTERN_C_END
  **/
 - (void)clear;
 
+/**
+ * Clears any unknown fields on this message.
+ *
+ * Note: To clear this message's unknown field and all the unknown fields of the
+ * messages within the fields of this message, use
+ * `GPBMessageDropUnknownFieldsRecursively()`.
+ **/
+- (void)clearUnknownFields;
+
 @end
 
 NS_ASSUME_NONNULL_END

objectivec/GPBMessage.m

@@ -1251,6 +1251,10 @@ static GPBUnknownFieldSet *GetOrMakeUnknownFields(GPBMessage *self) {
   }
 }
 
+- (void)clearUnknownFields {
+  self.unknownFields = nil;
+}
+
 - (BOOL)isInitialized {
   GPBDescriptor *descriptor = [self descriptor];
   for (GPBFieldDescriptor *field in descriptor->fields_) {

objectivec/GPBUtilities.m

@@ -63,7 +63,7 @@ void GPBMessageDropUnknownFieldsRecursively(GPBMessage *initialMessage) {
     [todo removeLastObject];
 
     // Clear unknowns.
-    msg.unknownFields = nil;
+    [msg clearUnknownFields];
 
     // Handle the message fields.
     GPBDescriptor *descriptor = [[msg class] descriptor];