Add Piper as TTS provider

[rkusa]

Add Piper as another TTS provider.

Similar to #231, it cannot directly be compiled into DCS-gRPC, but at least doesn't require running an http server. In a nutshell: it requires to manually download Piper, and DCS-gRPC runs the piper.exe each time TTS is executed.

I am not sure if I really want to add Piper this way, which is why I'll be keeping this as a draft PR for now. Ideally, I'd personally prefer to remove all other TTS options eventually, if we are able to settle on a local option with great quality (so that we/I don't have to maintain 5 different providers).

It definitely has great speech quality and does not require a lot of resources (it is optimized for the Raspberry PIs).

sample.zip

Voices can also be tested here: https://rhasspy.github.io/piper-samples/

[rurounijones]

I think having a high quality local TTS provider is worth any dependency niggles like having a separate exe (We could always bundle it) and there isn't too much latency.

I think merging this is a net positive!.