Implement missing client-side dial tone/ringback tone for VoLTE calls

Some carriers (such as Thailand's AIS) has stopped playing dial tone as part of the "early media" and requires us to play it locally. This means for those times, it will seem like dial tone is missing.

This is noticed by SailfishOS a few years back and has fix implemented in ofono-binder-plugin. However, it seems like we have to plug ourselves into that.

So seems like this will require 2 tasks:

1. Changes in our oFono VoLTE plugins that notify ofono-binder-plugin that a ringback tone is needed (this has to be implemented separately for QTI and MTK plugins).
2. A component that implements org.ofono.VoiceCallAgent interface, which get notified from oFono that a tone is needed. This interface is specific to the SailfishOS fork of oFono.