[roll third_party/pigweed] pw_rpc: Add packet processors to pwpb testing context

This CL adds an ability to customize packet processing in
pwpb ClientServerTestingContext (and its threaded variant).

This can be useful when the project has its own packet processing (e.g.
one that offloads all rpc handling to a custom thread pool) and wants to
simulate this behavior in tests as well.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/117091

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: fb4613037adcbcd..bf5301b649488e4
Roller-URL: https://ci.chromium.org/b/8779542630676622673
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I1efbf8e7e189ad9ed0638f3c6dfecdec5639d868
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/149815
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index fb46130..bf5301b 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit fb4613037adcbcdd0dec782c0fe0b8aeac79c36a
+Subproject commit bf5301b649488e4816858cb6a8daeb6131a074d3