[roll third_party/pigweed] pw_rpc: Do not hold locks while calling user code

- Add annotations and update code to ensure that rpc_lock() is not held
  while calling user-provided ChannelOutput functions.
- Flag a few places in the code with TODOs to fix thread safety issues.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/78120

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 3f126ecfc0651e2..a1c6bfd62b9dc23
Roller-URL: https://ci.chromium.org/b/8825303941420898993
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8825303941420898993
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I8aeaf34979494e8f887279609029d062e1a436c3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/78165
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 3f126ec..a1c6bfd 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 3f126ecfc0651e2a37a4f0e10b241c615d1057e6
+Subproject commit a1c6bfd62b9dc2343ada32cfc8a9cc724250a863