Add single asset vault (XLS-65d) (#5224)

- Specification: XRPLF/XRPL-Standards#239
- Amendment: `SingleAssetVault`
- Implements a vault feature used to store a fungible asset (XRP, IOU, or MPT, but not NFT) and to receive shares in the vault (an MPT) in exchange.
- A vault can be private or public.
- A private vault can use permissioned domains, subject to the `PermissionedDomains` amendment.
- Shares can be exchanged back into asset with `VaultWithdraw`.
- Permissions on the asset in the vault are transitively applied on shares in the vault.
- Issuer of the asset in the vault can clawback with `VaultClawback`.
- Extended `MPTokenIssuance` with `DomainID`, used by the permissioned domain on the vault shares.

Co-authored-by: John Freeman <jfreeman08@gmail.com>

include/xrpl/json/json_value.h

@@ -20,11 +20,13 @@
 #ifndef RIPPLE_JSON_JSON_VALUE_H_INCLUDED
 #define RIPPLE_JSON_JSON_VALUE_H_INCLUDED
 
+#include <xrpl/basics/Number.h>
 #include <xrpl/json/json_forwards.h>
 
 #include <cstring>
 #include <map>
 #include <string>
+#include <utility>
 #include <vector>
 
 /** \brief JSON (JavaScript Object Notation).
@@ -216,6 +218,7 @@ public:
     Value(UInt value);
     Value(double value);
     Value(char const* value);
+    Value(ripple::Number const& value);
     /** \brief Constructs a value from a static string.
 
      * Like other value string constructor but do not duplicate the string for
@@ -365,6 +368,8 @@ public:
      */
     Value&
     operator[](StaticString const& key);
+    Value const&
+    operator[](StaticString const& key) const;
 
     /// Return the member named key if it exist, defaultValue otherwise.
     Value
@@ -436,6 +441,12 @@ private:
     int allocated_ : 1;  // Notes: if declared as bool, bitfield is useless.
 };
 
+inline Value
+to_json(ripple::Number const& number)
+{
+    return to_string(number);
+}
+
 bool
 operator==(Value const&, Value const&);