pw_hdlc_lite: Build HDLC RPC example with CMake

- Add CMakeLists.txt for pw_hdlc_lite's RPC example.
- Update the pw_rpc CMake to split out individual libraries.
- Small docs update for using third party code in the CMake build.

Change-Id: I3ee8b85a275c3402dc494b0ea3a8140005241ccf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/24280
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_protobuf_compiler/proto.cmake b/pw_protobuf_compiler/proto.cmake
index 50e05e8..1966fc7 100644
--- a/pw_protobuf_compiler/proto.cmake
+++ b/pw_protobuf_compiler/proto.cmake
@@ -217,8 +217,8 @@
   target_link_libraries("${NAME}.nanopb_rpc"
     INTERFACE
       "${NAME}.nanopb"
+      pw_rpc.nanopb.method_union
       pw_rpc.server
-      pw_rpc.nanopb
       ${DEPS}
   )
   add_dependencies("${NAME}.nanopb_rpc" "${NAME}.generate.nanopb_rpc")