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/Indexes.h

@@ -287,6 +287,30 @@ did(AccountID const& account) noexcept;
 Keylet
 oracle(AccountID const& account, std::uint32_t const& documentID) noexcept;
 
+Keylet
+mptIssuance(std::uint32_t seq, AccountID const& issuer) noexcept;
+
+Keylet
+mptIssuance(MPTID const& issuanceID) noexcept;
+
+inline Keylet
+mptIssuance(uint256 const& issuanceKey)
+{
+    return {ltMPTOKEN_ISSUANCE, issuanceKey};
+}
+
+Keylet
+mptoken(MPTID const& issuanceID, AccountID const& holder) noexcept;
+
+inline Keylet
+mptoken(uint256 const& mptokenKey)
+{
+    return {ltMPTOKEN, mptokenKey};
+}
+
+Keylet
+mptoken(uint256 const& issuanceKey, AccountID const& holder) noexcept;
+
 }  // namespace keylet
 
 // Everything below is deprecated and should be removed in favor of keylets:
@@ -327,6 +351,9 @@ std::array<keyletDesc<AccountID const&>, 6> const directAccountKeylets{
      {&keylet::nftpage_max, jss::NFTokenPage, true},
      {&keylet::did, jss::DID, true}}};
 
+MPTID
+makeMptID(std::uint32_t sequence, AccountID const& account);
+
 }  // namespace ripple
 
 #endif