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/protocol/STInteger.cpp

@@ -199,12 +199,16 @@ Json::Value
 STUInt64::getJson(JsonOptions) const
 {
     auto convertToString = [](uint64_t const value, int const base) {
-        assert(base == 10 || base == 16);
+        ASSERT(
+            base == 10 || base == 16,
+            "ripple::STUInt64::getJson : base 10 or 16");
         std::string str(
             base == 10 ? 20 : 16, 0);  // Allocate space depending on base
         auto ret =
             std::to_chars(str.data(), str.data() + str.size(), value, base);
-        assert(ret.ec == std::errc());
+        ASSERT(
+            ret.ec == std::errc(),
+            "ripple::STUInt64::getJson : to_chars succeeded");
         str.resize(std::distance(str.data(), ret.ptr));
         return str;
     };