Add support for XLS-85 Token Escrow (#5185)

- Specification: https://github.com/XRPLF/XRPL-Standards/pull/272
- Amendment: `TokenEscrow`
- Enables escrowing of IOU and MPT tokens in addition to native XRP.
- Allows accounts to lock issued tokens (IOU/MPT) in escrow objects, with support for freeze, authorization, and transfer rates.
- Adds new ledger fields (`sfLockedAmount`, `sfIssuerNode`, etc.) to track locked balances for IOU and MPT escrows.
- Updates EscrowCreate, EscrowFinish, and EscrowCancel transaction logic to support IOU and MPT assets, including proper handling of trustlines and MPT authorization, transfer rates, and locked balances.
- Enforces invariant checks for escrowed IOU/MPT amounts.
- Extends GatewayBalances RPC to report locked (escrowed) balances.

include/xrpl/protocol/STAmount.h

@@ -703,6 +703,12 @@ isXRP(STAmount const& amount)
     return amount.native();
 }
 
+bool
+canAdd(STAmount const& amt1, STAmount const& amt2);
+
+bool
+canSubtract(STAmount const& amt1, STAmount const& amt2);
+
 // Since `canonicalize` does not have access to a ledger, this is needed to put
 // the low-level routine stAmountCanonicalize on an amendment switch. Only
 // transactions need to use this switchover. Outside of a transaction it's safe