proxy: add a payload field for the proxy options

I don't quite recall what we do in the other places where we use this,
but we should pass this payload to the callbacks.

include/git2/sys/remote.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_sys_git_transport_h
+#define INCLUDE_sys_git_transport_h
+
+#include "git2/net.h"
+#include "git2/types.h"
+
+GIT_BEGIN_DECL
+
+GIT_END_DECL