pw_status: Optionally add [[nodiscard]] to status

- Add a config option to pw_status that enables [[nodiscard]] to
  pw::Status and pw::StatusWithSize. Default to off.
- Add [[nodiscard]] to pw::Result.
- Add Abseil-style IgnoreError() functions to Status, StatusWithSize,
  and Result.

Change-Id: Iaa11ce5edc963e8bdd795e743545bafa9e42bf62
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/46603
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>