pw_rpc: Isolate pw_rpc server from proto library

- Completely isolate the pw_rpc server library from the protobuf
  implementation. The core pw_rpc server code compiles the same
  regardless of which method implementations are used. This opens up the
  possibility of using different protobuf libraries in the same server.
- No longer get pw_rpc/internal/method.h from the protobuf library
  implementation. Rename pw_rpc/internal/base_method.h to
  pw_rpc/internal/method.h and BaseMethod to Method.
- Rename the Nanopb method implementation to NanopbMethod and the test
  implementation to TestMethod.

Change-Id: I173edf75202159d3ce737f8029c9dc4a580f2cbe
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/18241
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
diff --git a/pw_protobuf_compiler/proto.gni b/pw_protobuf_compiler/proto.gni
index 9ddcfa3..7b32e80 100644
--- a/pw_protobuf_compiler/proto.gni
+++ b/pw_protobuf_compiler/proto.gni
@@ -162,7 +162,8 @@
     all_dependent_configs = [ ":$_include_config_target" ]
     deps = [ ":$_gen_target" ]
     public_deps = [
-                    "$dir_pw_rpc:nanopb_server",
+                    "$dir_pw_rpc:server",
+                    "$dir_pw_rpc/nanopb:method",
                     "$dir_pw_third_party/nanopb",
                   ] + invoker.gen_deps
     public = get_target_outputs(":$_gen_target")