Support for two-way audio

[nanosonde]

Homekit also supports video doorbells with two-way-audio. The "only" thing missing here is the return audio from Homekit to the Raspberry Pi. For this the speaker service must be added to the camera services.

Then we could use another ffmpeg instance to decode the audio from Homekit to the ALSA speaker device. I already did this here: https://github.com/nanosonde/homebridge-camera-ffmpeg/blob/sipdoorbell/src/streamingDelegate.ts#L516-L574

(The code above connects to a negotiated SIP RTP endpoint. This would not be required here as we could directly access the ALSA audio device.)