Add GPBMessageDropUnknownFieldsRecursively() and tests.
GPBMessageDropUnknownFieldsRecursively() is a new helper to drop the
unknownFields from a message and all sub messages (in fields or extensions).
diff --git a/objectivec/GPBUtilities.h b/objectivec/GPBUtilities.h
index 52e7d2e..5464dfb 100644
--- a/objectivec/GPBUtilities.h
+++ b/objectivec/GPBUtilities.h
@@ -392,6 +392,11 @@
**/
NSData *GPBEmptyNSData(void) __attribute__((pure));
+/**
+ * Drops the `unknownFields` from the given message and from all sub message.
+ **/
+void GPBMessageDropUnknownFieldsRecursively(GPBMessage *message);
+
NS_ASSUME_NONNULL_END
CF_EXTERN_C_END
