Add hooks for start/stop STT processing

[joshuaboniface]

I find it very useful to know via hook feedback when my Kalliope actually stops listening (i.e. the microphone is no longer actively recording) and when STT starts processing, especially since with some STT providers this latter step can take some amount of time.

This alters the behaviour of the old on_stop_listening hook to fire inside the STT Utils when the listener actually stops listening and the callback is fired, and then adds two new hooks around said callback: on_start_stt_processing and on_stop_stt_processing. These are optional like most others, but could be used if desired.

With this new setup, the on_start_listening/on_stop_listening only fire when the microphone is in use by the STT library; I can't see a good reason why this wouldn't happen, but am open to feedback if there is a better place to put these.

[joshuaboniface]

Should be good now, new tests passing successfully.

[Sispheor]

Thx

[joshuaboniface]

Glad to help!