add new audiotools.SpeedChanger module for WAV/MP3 speed changing

[todbot]

This adds support for adjustable playback speed / rate to audio streams (Wavefile, MP3Decoder, etc),
addressing issue #10881.

It is a reworking of the idea from #10882 to apply to more than just WaveFile.

New CIRCUITPY_AUDIOTOOLS definition added, only enabled for raspberrypi builds.

[tannewt]

Looks fine. My only gripe is that audiotools is vague and likely to have other stuff thrown into it. I'd prefer something more specific like audiospeed (which better matches audiodelays). Are you ok with that change? Claude should make quick work of it.

[todbot]

Yeah audiospeed is fine, if a bit fine-grained. I can make the change.
I was struggling for a good top-level name to indicate what SpeedChanger was just one of: a class of audio stream wrappers that weren't effects but low-cost transformers or utilities. Like stereo<->mono swapper, L-R center-cut, phase invert. I'm not sure how useful those would be in CircuitPython, so I don't have a strong hold on audiotools.

[tannewt]

Looks good! Thank you!