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>

include/xrpl/protocol/Issue.h

@@ -38,16 +38,26 @@ public:
     Currency currency{};
     AccountID account{};
 
-    Issue()
-    {
-    }
+    Issue() = default;
 
     Issue(Currency const& c, AccountID const& a) : currency(c), account(a)
     {
     }
 
+    AccountID const&
+    getIssuer() const
+    {
+        return account;
+    }
+
     std::string
     getText() const;
+
+    void
+    setJson(Json::Value& jv) const;
+
+    bool
+    native() const;
 };
 
 bool
@@ -116,6 +126,12 @@ noIssue()
     return issue;
 }
 
+inline bool
+isXRP(Issue const& issue)
+{
+    return issue.native();
+}
+
 }  // namespace ripple
 
 #endif