session: add vnet_connect_stream

session: add vnet_connect_stream

Ask transport to open stream on existing connection, stream must be
opened on same thread as parent connection. For internall
applications stream is opened instantaneously.
Transport protocl must implement connect_stream callback.
This is useful for protocols like HTTP/2, HTTP/3 or QUIC which use
streams to provide lightweight, ordered byte-stream abstraction to an
application, they are created by sending data.

Type: improvement

Change-Id: I4b6b5a003a6e1c56135cb26e067c42956ba5ae06
Signed-off-by: Matus Fabian <[email protected]>