Support Token Escrows

[sappenin]

Add XLS-0085 Token Escrow Support (v6.0.0)

Overview

This PR implements support for the XLS-0085 Token Escrow amendment, which extends the existing Escrow functionality to support Trustline-based tokens (IOUs)** and **Multi-Purpose Tokens (MPTs) in addition to XRP.

✨ New Features

1. IOU Token Escrows

• IOU tokens can now be held in escrow
• Requires issuer to enable the new ALLOW_TRUSTLINE_LOCKING flag
• Transfer rates are locked at escrow creation time

2. MPT Token Escrows

• MPT tokens can now be held in escrow
• Requires lsfMPTCanEscrow and lsfMPTCanTransfer flags on the token
• Locked amounts are tracked in MpTokenObject and MpTokenIssuanceObject

3. New AccountSet Flag

• Added ALLOW_TRUSTLINE_LOCKING(17) enum value
• Added AccountRootFlags.ALLOW_TRUSTLINE_LOCKING (0x40000000) constant
• Added lsfAllowTrustLineLocking() method for checking account state

4. New EscrowObject Fields

• transferRate(): Stores the locked transfer rate (IOU) or transfer fee (MPT) at escrow creation
• issuerNode(): Reference to issuer's directory node when issuer is neither source nor destination

5. MPT Locked Amount Tracking

• MpTokenObject.lockedAmount(): Tracks amount locked in escrows for a holder
• MpTokenIssuanceObject.lockedAmount(): Tracks total amount locked across all holders

✅ Test Coverage

• Unit Tests: 168/168 passing (10 new tests added)
• Integration Tests: Full coverage for IOU and MPT escrow scenarios including create/finish/cancel operations, flag handling, transfer rate locking, and error conditions

🚨 Breaking Changes

While all existing XRP escrow functionality remains fully backward compatible, there are some breaking changes required to support IOU and MPT tokens.

Type Changes

• EscrowCreate.amount(): XrpCurrencyAmount → CurrencyAmount
• EscrowObject.amount(): XrpCurrencyAmount → CurrencyAmount
• MetaEscrowObject.amount(): XrpCurrencyAmount → CurrencyAmount

Impact: Code that reads escrow amounts must be updated to handle CurrencyAmount using .handle() or .map() methods. Building transactions with XrpCurrencyAmount continues to work due to polymorphism.

[sappenin]

Note: This is typed as a String to conform to the Hex value returned by the RPC layer. Also aligns with RippleStateObject types like lowNode and highNode`.