Add STInt32 as a new SType (#5788)

This change adds `STInt32` as a new `SType` under the `STInteger` umbrella, with `SType` value `12`. This is the first and only `STInteger` type that supports negative values.

src/test/protocol/STAccount_test.cpp

@@ -122,10 +122,27 @@ struct STAccount_test : public beast::unit_test::suite
         }
     }
 
+    void
+    testAccountID()
+    {
+        auto const s = "rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh";
+        if (auto const parsed = parseBase58<AccountID>(s); BEAST_EXPECT(parsed))
+        {
+            BEAST_EXPECT(toBase58(*parsed) == s);
+        }
+
+        {
+            auto const s =
+                "âabcd1rNxp4h8apvRis6mJf9Sh8C6iRxfrDWNâabcdAVâ\xc2\x80\xc2\x8f";
+            BEAST_EXPECT(!parseBase58<AccountID>(s));
+        }
+    }
+
     void
     run() override
     {
         testSTAccount();
+        testAccountID();
     }
 };