pw_analog: Add analog::AnalogInput interface

Adds the pw_analog module with the pw::analog::AnalogInput interface for
sampling the ADC to get a voltage reading.

Testing:
Host test -- OK

Change-Id: Ief497da89e34eec6aa1be4c3f29c484a85a39cfb
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/38601
Commit-Queue: Kevin Zeng <zengk@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>