[roll third_party/pigweed] pw_rpc: Responder & testing improvements

- Fill out remaining server-side Nanopb client & bidirectional streaming
  features and add tests.
- Use MethodType instead of a custom HasClientStream in Responder.
- Expose RawFakeChannelOutput and NanopbFakeChannelOutput for testing
  use.
- Reorganize test utils headers to avoid the need to add "." as an
  include path.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/57881

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 81504c443d5c0f2..fa6edcc11505218
Roller-URL: https://ci.chromium.org/b/8837800692751562737
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8837800692751562737
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I77a512b214298f32a9638ea1520cc6b1fef479a0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/58500
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 81504c4..fa6edcc 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 81504c443d5c0f2ab88c6ed383dbbd4d757bfe21
+Subproject commit fa6edcc1150521826a6f94f397658685b4988859