MEDIUM: stconn: Add functions to handle applets I/O from the SC layer

MEDIUM: stconn: Add functions to handle applets I/O from the SC layer

There is no tasklet to handle I/O subscriptions for applets, but functions
to deal with receives and sends from the SC layer were added. it meanse a
function to retrieve data from an applet with this synchronous version and a
function to push data to an applet wit this synchronous version.

It is pretty similar to the functions used for muxes but there are some
differences. So for now, we keep them separated.

Zero-copy forwarding is not supported for now. In addition, there is no
subscription mechanism.