Submits extension frame.

Application can pass arbitrary frame flags and stream ID in *flags*
and *stream_id* respectively.  The *payload* is opaque pointer, and
it can be accessible though ``frame->ext.payload`` in
:type:`nghttp2_pack_extension_callback2`.  The library will not own
passed *payload* pointer.

The application must set :type:`nghttp2_pack_extension_callback2`
using `nghttp2_session_callbacks_set_pack_extension_callback2()`.

The application should retain the memory pointed by *payload* until
the transmission of extension frame is done (which is indicated by
:type:`nghttp2_on_frame_send_callback`), or transmission fails
(which is indicated by :type:`nghttp2_on_frame_not_send_callback`).
If application does not touch this memory region after packing it
into a wire format, application can free it inside
:type:`nghttp2_pack_extension_callback2`.

The standard HTTP/2 frame cannot be sent with this function, so
*type* must be strictly grater than 0x9.  Otherwise, this function
will fail with error code
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`.

This function returns 0 if it succeeds, or one of the following
negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
    If :type:`nghttp2_pack_extension_callback2` is not set.
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
    If  *type* specifies  standard  HTTP/2 frame  type.  The  frame
    types  in the  rage [0x0,  0x9], both  inclusive, are  standard
    HTTP/2 frame type, and cannot be sent using this function.
:enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
    Out of memory