[roll third_party/pigweed] pw_tokenizer: Support alternate prefix in protos

Updates pw_tokenizer.proto to support optionally tokenized fields that
may use alternate prefixes. Updates docs to highlight how to detokenize
optionally tokenized messages in the Python detokenization section.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/53101

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 2fd417afdfe524f..dd646718bf1893e
Roller-URL: https://ci.chromium.org/b/8839263549081022161
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8839263549081022161
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If2b1cd4a63e0c6aab02febf15d1b10725e3ec023
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/56740
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 2fd417a..dd64671 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 2fd417afdfe524f7cacbb68e54e8113f32af9165
+Subproject commit dd646718bf1893e13edeffbacb23b665ed45a5e9