[roll third_party/pigweed] pw_rpc: Python client & C++ server integration test

- Create utilties for interacting with a host binary running a pw_rpc
  server using HDLC over a socket.
- Write a test that runs commands in a Python client on a C++ server.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/54665

third_party/pigweed Rolled-Commits: 2a5f4da1be0bbb3..e04d4687944e0a1
Roller-URL: https://ci.chromium.org/b/8839852994719176945
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8839852994719176945
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id6b17fe17f98a0b6e6bdc6271c84802e6e072076
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/56034
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 2a5f4da..e04d468 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 2a5f4da1be0bbb38b3cf8f2d31e203cf86309b7c
+Subproject commit e04d4687944e0a1e8f9e61c206d644ad7db987f2