Add support for NSS (Mac OS built-in TTS)

[gbottari]

Description

Since we are using pyttsx3 we could easily support that engine as well.

Suggested Steps

1. Create the client and TTS classes
2. Add tests following the base test classes
3. Mark the tests so it only runs on Mac OS
4. Actually test on Mac OS
5. Bonus: see if we can have it working on the CI Runner (probably not possible)

[willwade]

NB: https://github.com/Jiangshan00001/pyttsx4 does this pretty well