split out handler implementation from interface

Split out the handler object definition from the interface and drop the
default parameters.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ie8c9dbf7881bbe6b5af3596a60430e3b3d573762
diff --git a/test/handler_unittest.cpp b/test/handler_unittest.cpp
index a4f9281..99782fa 100644
--- a/test/handler_unittest.cpp
+++ b/test/handler_unittest.cpp
@@ -1,5 +1,6 @@
 #include "errors.hpp"
 #include "handler.hpp"
+#include "handler_impl.hpp"
 
 #include <fstream>
 #include <nlohmann/json.hpp>