Change HTTPCallback::OnReceiveData to use UniqueBuffer

See: https://github.com/OpenTTD/OpenTTD/issues/11636

src/network/network_content.h

@@ -97,7 +97,7 @@ protected:
 	void OnDownloadComplete(ContentID cid) override;
 
 	void OnFailure() override;
-	void OnReceiveData(const char *data, size_t length) override;
+	void OnReceiveData(UniqueBuffer<char> data) override;
 	bool IsCancelled() const override;
 
 	bool BeforeDownload();