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_rpc/service_test.cc b/pw_rpc/service_test.cc
index 2fadec7..107ad78 100644
--- a/pw_rpc/service_test.cc
+++ b/pw_rpc/service_test.cc
@@ -15,27 +15,27 @@
 #include "pw_rpc/service.h"
 
 #include "gtest/gtest.h"
-#include "pw_rpc/internal/base_method.h"
+#include "pw_rpc/internal/method.h"
 
 namespace pw::rpc {
 
 class ServiceTestHelper {
  public:
-  static const internal::BaseMethod* FindMethod(Service& service, uint32_t id) {
+  static const internal::Method* FindMethod(Service& service, uint32_t id) {
     return service.FindMethod(id);
   }
 };
 
 namespace {
 
-void InvokeIt(const internal::BaseMethod&,
+void InvokeIt(const internal::Method&,
               internal::ServerCall&,
               const internal::Packet&) {}
 
-class ServiceTestMethod : public internal::BaseMethod {
+class ServiceTestMethod : public internal::Method {
  public:
   constexpr ServiceTestMethod(uint32_t id, char the_value)
-      : internal::BaseMethod(id, InvokeIt), value(the_value) {}
+      : internal::Method(id, InvokeIt), value(the_value) {}
 
   char value;  // Add a member so the class is larger than the base Method.
 };