[roll third_party/pigweed] pw_rpc: Make MethodType public

- MethodType will be used in testing APIs and is harmless to have be
  public. Move it from pw::rpc::internal to pw::rpc.
- Add HasServerStream and HasClientStream functions that check if a
  MethodType includes server or client streaming components.
- Add method_test to cover the generic Method class and the MethodType
  enum.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/56980

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: b8c67ac97bec39a..1a4478bc3c621ce
Roller-URL: https://ci.chromium.org/b/8839039856393748385
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8839039856393748385
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iad60ceb564f589c06ba903e634832b69dbbe8861
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/56982
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 b8c67ac..1a4478b 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit b8c67ac97bec39a50ead4f976f7754d53daad500
+Subproject commit 1a4478bc3c621ce74f192dac3a0663cec71b3f88