Add Antithesis intrumentation (#5042)

* Copy Antithesis SDK version 0.4.0 to directory external/
* Add build option `voidstar` to enable instrumentation with Antithesis SDK
* Define instrumentation macros ASSERT and UNREACHABLE in terms of regular C assert
* Replace asserts with named ASSERT or UNREACHABLE
* Add UNREACHABLE to LogicError
* Document instrumentation macros in CONTRIBUTING.md

src/libxrpl/json/json_reader.cpp

@@ -953,7 +953,7 @@ operator>>(std::istream& sin, Value& root)
     Json::Reader reader;
     bool ok = reader.parse(sin, root);
 
-    // JSON_ASSERT( ok );
+    // ASSERT(ok, "Json::operator>>() : parse succeeded");
     if (!ok)
         ripple::Throw<std::runtime_error>(reader.getFormatedErrorMessages());