refactor(nm): add NetworkManagerDbusWrapper and ModemManagerDbusWrapper objects

[mattdibi]

This PR adds two new objects to the org.eclipse.kura.nm bundle that abstract the two main services to which the NMDbusConnector interfaces: NetworkManager and ModemManager.

This refactoring was necessary since the NMDbusConnector, initially dedicated to interfacing only with NetworkManager, grew too much and started touching other services (ModemManager, soon WPA Supplicant) as features were added.

The introduction of these two new objects should increase the readability of the code since it is now clear to which dbus service the NMDbusConnector is talking to.

Additional notes:

• I moved getModemDeviceHwPath method from NMStatusConverter to the ModemManager wrapper.

Things I don't particularly like:

• The NetworkManagerDbusWrapper and the ModemManagerDbusWrapper have a dbusConnection as private member, but the sole owner of the dbusConnection is NMDbusConnector (which is why we modeled it as a singleton). Another way to avoid confusion would be to always pass the dbusConnection as parameter of the method. Any suggestion is welcome.
• The modem reset handlers were moved inside ModemManagerDbusWrapper since ModemManager is responsible for managing the Modem but to decide whether we need to reset the modem or not we monitor a NetworkManager signal. The reset handlers are a "bridge" between the two services but I decided to move them in the ModemManager anyway.
• Lack of unit tests for the wrappers object. Coverage is pretty high because we have the NMDbusConnector tests but a set of dedicated tests for the wrappers would be a great idea.

Test performed on Raspberry Pi 4:

• Installation and check that all connection work as expected
• Wifi connection
• Cellular connection
• Modem reset
• Configuration enforcement
• Manual IP configuration for ethernet device

Any additional test is welcome