Add hw-adc gem with analog-to-digital conversion support:
- Common Ruby API with read, read_raw, and read_voltage
- ports/esp32/ using ESP-IDF ADC oneshot driver
- ports/rp2040/ using Pico SDK ADC hardware
Based on picoruby-adc by HASUMI Hitoshi.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto