MINOR: stconn: Add samples to retrieve about stream aborts

MINOR: stconn: Add samples to retrieve about stream aborts

It is now possible to retrieve some info about the abort received for a
server or a client stream, if any.

  * fs.aborted and bs.aborted can be used to know if an abort was received
    on frontend or backend side. A boolean is returned.

  * fs.rst_code and bs.rst_code return the code of the received RESET_STREAM
    frame for a H2 stream or the code of the received STOP_SENDING frame for
    a QUIC stream. In both cases, the error code attached to the frame is
    returned. The sample fetch fails if no such frame was received or if the
    stream is not an H2/QUIC stream.