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/basics/Number.h

@@ -20,6 +20,7 @@
 #ifndef RIPPLE_BASICS_NUMBER_H_INCLUDED
 #define RIPPLE_BASICS_NUMBER_H_INCLUDED
 
+#include <xrpl/basics/MPTAmount.h>
 #include <xrpl/basics/XRPAmount.h>
 #include <cstdint>
 #include <limits>
@@ -52,6 +53,7 @@ public:
     explicit constexpr Number(rep mantissa, int exponent, unchecked) noexcept;
 
     Number(XRPAmount const& x);
+    Number(MPTAmount const& x);
 
     constexpr rep
     mantissa() const noexcept;
@@ -88,9 +90,16 @@ public:
     static constexpr Number
     lowest() noexcept;
 
+    /** Conversions to Number are implicit and conversions away from Number
+     *  are explicit. This design encourages and facilitates the use of Number
+     *  as the preferred type for floating point arithmetic as it makes
+     *  "mixed mode" more convenient, e.g. MPTAmount + Number.
+     */
     explicit
     operator XRPAmount() const;  // round to nearest, even on tie
     explicit
+    operator MPTAmount() const;  // round to nearest, even on tie
+    explicit
     operator rep() const;  // round to nearest, even on tie
 
     friend constexpr bool
@@ -212,6 +221,10 @@ inline Number::Number(XRPAmount const& x) : Number{x.drops()}
 {
 }
 
+inline Number::Number(MPTAmount const& x) : Number{x.value()}
+{
+}
+
 inline constexpr Number::rep
 Number::mantissa() const noexcept
 {