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/STTx.cpp

@@ -137,7 +137,9 @@ STTx::getMentionedAccounts() const
     {
         if (auto sacc = dynamic_cast<STAccount const*>(&it))
         {
-            assert(!sacc->isDefault());
+            ASSERT(
+                !sacc->isDefault(),
+                "ripple::STTx::getMentionedAccounts : account is set");
             if (!sacc->isDefault())
                 list.insert(sacc->value());
         }
@@ -298,7 +300,8 @@ STTx::getMetaSQL(
     std::string rTxn = sqlBlobLiteral(rawTxn.peekData());
 
     auto format = TxFormats::getInstance().findByType(tx_type_);
-    assert(format != nullptr);
+    ASSERT(
+        format != nullptr, "ripple::STTx::getMetaSQL : non-null type format");
 
     return str(
         boost::format(bfTrans) % to_string(getTransactionID()) %