Add buffer read to AudioBufferManager and I2S.

[relic-se]

I quickly noticed after our discussion in #2775 that a buffered read was not available in the I2S library or the AudioBufferManager. I've added the necessary functionality to do this as well as a second loopback example demonstrating how to properly implement it. The documentation has also been updated with this method.

Notes:

• I have noticed performance improvements with this feature properly implemented.
• I've fixed an error in the documentation regarding the return value of size_t I2S::write(const uint8_t *buffer, size_t size). I'm good with having that function and the new size_t I2S::write(const uint8_t *buffer, size_t size) return the number of words written. But to match the nature of the size_t size argument (size in bytes), the return could be scaled by sizeof(uint32_t).

[earlephilhower]

Minor const question, but OTW thanks very much for the contribution! I focused my effort on making the output path faster since I use it for https://github.com/earlephilhower/BackgroundAudio and https://github.com/earlephilhower/ESP8266Audio so you're hitting these bottlenecks. Thx again for taking the time to help alleviate them!

[relic-se]

@earlephilhower Everything should be good to go now. Thanks for the review!

[earlephilhower]

Great, thx!

[relic-se]

@earlephilhower Thanks for catching that last const!

[earlephilhower]

No worries. C casting footgun avoided. Probably others in my code still lurking... Thanks again!