Introduce MPT support (XLS-33d): (#5143)

Amendment:
- MPTokensV1

New Transactions:
- MPTokenIssuanceCreate
- MPTokenIssuanceDestroy
- MPTokenIssuanceSet
- MPTokenAuthorize

Modified Transactions:
- Payment
- Clawback

New Objects:
- MPTokenIssuance
- MPToken

API updates:
- ledger_entry
- account_objects
- ledger_data

Other:
- Add += and -= operators to ValueProxy

Read full spec: https://github.com/XRPLF/XRPL-Standards/tree/master/XLS-0033d-multi-purpose-tokens

---------
Co-authored-by: Shawn Xie <shawnxie920@gmail.com>
Co-authored-by: Howard Hinnant <howard.hinnant@gmail.com>
Co-authored-by: Ed Hennis <ed@ripple.com>
Co-authored-by: John Freeman <jfreeman08@gmail.com>

src/libxrpl/protocol/STInteger.cpp

@@ -198,11 +198,23 @@ template <>
 Json::Value
 STUInt64::getJson(JsonOptions) const
 {
-    std::string str(16, 0);
-    auto ret = std::to_chars(str.data(), str.data() + str.size(), value_, 16);
-    assert(ret.ec == std::errc());
-    str.resize(std::distance(str.data(), ret.ptr));
-    return str;
+    auto convertToString = [](uint64_t const value, int const base) {
+        assert(base == 10 || base == 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());
+        str.resize(std::distance(str.data(), ret.ptr));
+        return str;
+    };
+
+    if (auto const& fName = getFName(); fName.shouldMeta(SField::sMD_BaseTen))
+    {
+        return convertToString(value_, 10);  // Convert to base 10
+    }
+
+    return convertToString(value_, 16);  // Convert to base 16
 }
 
 }  // namespace ripple