roll: third_party/pigweed pw_channel: Introduce pw::channel::PacketChannel

PacketReader, PacketWriter, and PacketReaderWriter support reading
and/or writing typed packets.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/293512
Original-Revision: 4261c539e48bfb293cda805ffc9d436175aac88f

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: caa9c9b10bf843..4261c539e48bfb
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8711124143461678113
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3219a1072b0baf6894ddbaa3b70ee0f58b6e0614
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/300512
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index caa9c9b..4261c53 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit caa9c9b10bf8430ca075e1d500581d55d4e62c9d
+Subproject commit 4261c539e48bfb293cda805ffc9d436175aac88f