Initial support for secrets in gw:

- added SecretHandler type
- added discussed system/secret endpoint with appropriate http verbs

Signed-off-by: Andrew Cornies <acornies@gmail.com>

gateway/types/handler_set.go

@@ -26,4 +26,7 @@ type HandlerSet struct {
 
 	// InfoHandler provides version and build info
 	InfoHandler http.HandlerFunc
+
+	// SecretHandler allows secrets to be managed
+	SecretHandler http.HandlerFunc
 }