plugins: Plugin for finding MNC length from IMSI

This plugin finds the MNC length from the IMSI using the valid MCC/MNC
pairs published by the ITU-T.
include: MNC length from IMSI driver interface
src: Auxiliar functions for MNC length plugins
[ofono] Add MNC plugin changes from Canonical

Signed-off-by: Miia Leinonen <miia.leinonen@oss.tieto.com>
build: Added mnclenght plugin and driver