Submits PUSH_PROMISE frame.

The *flags* is currently ignored.  The library handles the
CONTINUATION frame internally and it correctly sets END_HEADERS to
the last sequence of the PUSH_PROMISE or CONTINUATION frame.

The *stream_id* must be client initiated stream ID.

The *nva* is an array of name/value pair :type:`nghttp2_nv` with
*nvlen* elements.  The application is responsible to include
required pseudo-header fields (header field whose name starts with
":") in *nva* and must place pseudo-headers before regular header
fields.

This function creates copies of all name/value pairs in *nva*.  It
also lower-cases all names in *nva*.  The order of elements in
*nva* is preserved.  For header fields with
:enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
:enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
header field name and value are not copied respectively.  With
:enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
is responsible to pass header field name in lowercase.  The
application should maintain the references to them until
:type:`nghttp2_on_frame_send_callback` or
:type:`nghttp2_on_frame_not_send_callback` is called.

The *promised_stream_user_data* is a pointer to an arbitrary data
which is associated to the promised stream this frame will open and
make it in reserved state.  It is available using
`nghttp2_session_get_stream_user_data()`.  The application can
access it in :type:`nghttp2_before_frame_send_callback` and
:type:`nghttp2_on_frame_send_callback` of this frame.

The client side is not allowed to use this function.

To submit response headers and data, use
`nghttp2_submit_response2()`.

This function returns assigned promised stream ID if it succeeds,
or one of the following negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
    Out of memory.
:enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
    This function was invoked when *session* is initialized as
    client.
:enum:`nghttp2_error.NGHTTP2_ERR_STREAM_ID_NOT_AVAILABLE`
    No stream ID is available because maximum stream ID was
    reached.
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
    The *stream_id* is 0; The *stream_id* does not designate stream
    that peer initiated.
:enum:`nghttp2_error.NGHTTP2_ERR_STREAM_CLOSED`
    The stream was already closed; or the *stream_id* is invalid.

.. warning::

  This function returns assigned promised stream ID if it succeeds.
  As of 1.16.0, stream object for pushed resource is created when
  this function succeeds.  In that case, the application can submit
  push response for the promised frame.

  In 1.15.0 or prior versions, pushed stream is not opened yet when
  this function succeeds.  The application must not submit frame to
  that stream ID before :type:`nghttp2_before_frame_send_callback`
  is called for this frame.