[uri] add DIAG_GET URIs for disgnostic APIs (#267)

In order to enable libotcommissioner.jar to support the diagnostic
commands, it is necessary to integrate all diagnostic URIs into the uri
module.

src/library/uri.hpp

@@ -75,6 +75,14 @@ static const char *const kJoinEnt             = "/c/je";
 static const char *const kJoinFin             = "/c/jf";
 static const char *const kJoinApp             = "/c/ja";
 
+/*
+ * Thread diagnostic URIs
+ */
+static const char *const kDiagGet      = "/d/dg";
+static const char *const kDiagGetQuery = "/d/dq";
+static const char *const kDiagGetAns   = "/d/da";
+static const char *const kDiagRst      = "/d/dr";
+
 /*
  * Thread Network Layer URIs
  */