[roll third_party/pigweed] pw_rpc: Support asynchronous unary RPCs

- Implement asynchronous unary RPCs for the server. Define
  ServerResponder classes as the ServerWriter equivalent for unary RPCs.
- Use the responder classes instead of custom logic for synchronous
  unary RPCs.
- Expand tests to cover asynchronous unary RPCs.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/60400

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 73687fb473489a2..5ebdaeb1d246dc8
Roller-URL: https://ci.chromium.org/b/8836154791015341217
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8836154791015341217
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic8b5f591ffaaf013dc3c110362a349cefe81166a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/60698
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 73687fb..5ebdaeb 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 73687fb473489a203d63d9b0da3ad8ace82498ce
+Subproject commit 5ebdaeb1d246dc8f1bff83a35b733a63121322c1