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.

Change-Id: I3dd59ca593bd6ddd311d91a3de7f1b7eb60e3214
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/57881
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
diff --git a/pw_rpc/nanopb/CMakeLists.txt b/pw_rpc/nanopb/CMakeLists.txt
index 6f879f7..4859a91 100644
--- a/pw_rpc/nanopb/CMakeLists.txt
+++ b/pw_rpc/nanopb/CMakeLists.txt
@@ -17,6 +17,7 @@
 pw_add_module_library(pw_rpc.nanopb.method
   SOURCES
     method.cc
+    server_reader_writer.cc
   PUBLIC_DEPS
     pw_rpc.nanopb.common
     pw_rpc.server