I2s input API and examples (#4539)

Enables I2S stereo input via DMA using new API calls:

. i2s_rxtx_begin(bool rx, rool tx);
. i2s_read_sample(uint32_t *l, uint32_t *r);

Original API calls will only enable TX, so this is backwards compatible.

Add simple I2S input example code using Arduino serial plotter.

Add UDP transmit of I2S microphone data to a PC (remote microphone).

Clean up and reorganize code to share RX and TX logic as much as
possible.  Fix a potential WDT error while in blocking sample read
and write.

Author: earlephilhower