pw_analog: Add MicrovoltInput class

Add interface for converting an analog sample into a fixed point
voltage in microvolts with MicrovoltInput.

Testing:
Host test -- OK

Change-Id: I481e78ed99e170f6b3838a5ce283e869be810ee5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/43840
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: David Rogers <davidrogers@google.com>
Commit-Queue: Kevin Zeng <zengk@google.com>