MINOR: mux-quic/h3: define stream close callback

MINOR: mux-quic/h3: define stream close callback

Define a new qcc_app_ops callback named close(). This will be used to
notify app-layer about the closure of a stream by the remote peer. Its
main usage is to ensure that the closure is allowed by the application
protocol specification.

For the moment, close is not implemented by H3 layer. However, this
function will be mandatory to properly reject a STOP_SENDING on the
control stream and preventing a later crash. As such, this commit must
be backported with the next one on 2.6.

This is related to github issue #2006.