Add background thread abstraction

[ohmtech-rdi]

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Add a "background thread" abstraction, for tasks that are not suited to the audio realtime task, like loading a sample from SD.

The abstraction would allow to run on both the Daisy and simulator ERB targets.

• [ ] Add "background thread" abstraction to Daisy ERB target, using "main thread"
• [ ] Add "background thread" abstraction to Simulator ERB target, using std::thread

Note: any kind of synchronisation is out of the scope of this feature.