modem: add modem_pipelink module

Add modem pipelink module which exposes modem pipes globally.
The pipelink module implements a callback to inform when a
pipe becomes available to use by whichever modem is attached
to it. This could be a shell, or a network interface.

The module aims to allow modem drivers to be split into modules,
and allowing applications to implement their own custom logic
without altering the modem drivers.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>