Introduce proto -> pure c++ code generator

It will be used to generate pure C++ headers / translation units
from trace_config.proto, data_source_config.proto and
data_source_descriptor.proto.
This allows to keep in sync the .proto files and the C++ classes
in tracing/core more easily.
The end goal is to allow clients of the tracing/core API to not
have to depend on libprotobuf (unless their source TraceConfig is
a proto and they want to have that dependency deliberately).

Change-Id: I0fdc53071661bba14af867d01d0a001accc082dc
diff --git a/BUILD.gn b/BUILD.gn
index 7db89ed..962db7b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -22,6 +22,7 @@
     "src/ipc/protoc_plugin:ipc_plugin($host_toolchain)",
     "src/protozero/protoc_plugin($host_toolchain)",
     "tools/ftrace_proto_gen:ftrace_proto_gen",
+    "tools/proto_to_cpp",
   ]
   if (!build_with_chromium) {
     deps += [