samples: add sample and test for pw_tokenizer Include a sample that encodes and decodes a simple string Bug: b/236263182 Change-Id: If10be9360e7ef13e552b49e36853af634577dc42 Signed-off-by: Yuval Peress <peress@google.com> Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/109476 Reviewed-by: Wyatt Hepler <hepler@google.com>