Add callback for response handling to HttpClient

Adds sendDataWithCallback() which allows the caller to include a
callback specifying how to handle the response to that
request.  This will be utilized for Redfish Aggregation
including returning the responses received when forwarding
requests to satellite BMCs.

Change-Id: I93826c8b254a5f28a982295d4145453352a90fae
Signed-off-by: Carson Labrado <clabrado@google.com>
diff --git a/http/http_response.hpp b/http/http_response.hpp
index 58a8029..6c842d7 100644
--- a/http/http_response.hpp
+++ b/http/http_response.hpp
@@ -76,7 +76,7 @@
         return stringResponse->result();
     }
 
-    unsigned resultInt()
+    unsigned resultInt() const
     {
         return stringResponse->result_int();
     }