This fixes #360 and #356 and #355 and #353

Summary
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- Renamed `pa_audio_back_end` to `port_audio_back_end`.
- Created `SoundStreamSettings` struct to hold `samples_per_second`, `frames` and `channels`.
- Removed `soundstreamer` and `StreamPA`'s run method - replaced with `.run(settings: SoundStreamSettings)` method in `SoundStream`.
- Fixed example to take on new changes.