New Multi-SIM management APIs and Single-Standby support in QMI modems

This MR introduces new APIs to support multi-SIM setups. The new APIs are generic enough to support both single-standby (only one SIM may be active at a time) and multi-standby (multiple SIMs may be active at a time) BUT the implementation only covers single-standby support in QMI modems.

Introducing multi-standby support for QMI modems (e.g. by allowing to select which slot is used as "primary-GW" provisioning session) shouldn't be hard, but so far the most common setup for plain modems is single-standby.

Fixes: #186 (closed)