[cdc_stream] Automatically start service (#28)

Starts the streaming service if it's not up and running. This required
adding the ability to run a detached process. By default, all child
processes are killed when the parent process exits. Since detached
child processes don't run with a console, they need to create sub-
processes with CREATE_NO_WINDOW since otherwise a new console pops up,
e.g. for every ssh command.

Polls for 20 seconds while the service starts up. For this purpose,
a BackgroundServiceClient is added. This will be reused in a future CL
by a new stop-service command to exit the service.

Also adds --service-port as additional argument to start-service.

cdc_stream/local_assets_stream_manager_service_impl.cc

@@ -277,6 +277,7 @@ absl::Status LocalAssetsStreamManagerServiceImpl::InitSsh(
         absl::StrFormat(" --organization %s", Quoted(organization_id));
   }
   start_info.name = "ggp ssh init";
+  start_info.flags = ProcessFlags::kNoWindow;
 
   std::string output;
   start_info.stdout_handler = [&output, this](const char* data,