modem: backend: Add statistics module

Add statistics module which tracks the usage of modem backend
transmit/receive buffers if implemented by a backend.

A modem backend may register itself to the modem backend
stat module, and advertise its transmit and receive buffer
sizes and momentary usage (length).

The modem backend module tracks the maximum usage, which can be
output to the shell using the "modem_stats buffer" shell command.

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