[roll third_party/pigweed] Zephyr implementation of pw_rpc and dependencies

Provide a Zephyr implementation of the pigweed modules and facades
required for using pw_rpc in a Zephyr application.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/67186

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 6f6b6a12665eaac..b8f3ad2db9ad0f0
Roller-URL: https://ci.chromium.org/b/8827642435200882129
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8827642435200882129
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib26f8a294665d1dfada7f20a4f07a90c1c19f677
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/75320
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 6f6b6a1..b8f3ad2 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 6f6b6a12665eaac4854c6399e919736a9e32778c
+Subproject commit b8f3ad2db9ad0f08970d62af3910f91ee5389e12