Add binary codec and transaction models for MPT

[e-desouza]

High Level Overview of Change

Full MPT (Multi-Purpose Token) support for the binary codec and transaction/ledger models:

• New Hash192 type in the binary codec (24-byte fixed-length hash for MPTokenIssuanceID)
• Updated definitions.json with MPT transaction types, ledger entry types, and field definitions
• Four new transaction models: MPTokenIssuanceCreate, MPTokenIssuanceDestroy, MPTokenIssuanceSet, MPTokenAuthorize
• Two new ledger objects: MPToken, MPTokenIssuance
• 40 tests covering field encoding, type codes, transaction serialization, signing prefixes, and serde roundtrips

Closes #119

Context of Change

The XRPL MPT amendment introduces four new transaction types (codes 54-57) and two new ledger entry types (codes 126-127). This required:

1. A new Hash192 type -- MPTokenIssuanceID is a 24-byte (192-bit) hash, which didn't exist in the codec. Followed the same pattern as Hash128/Hash160/Hash256.

2. Seven new field definitions in definitions.json: MPTokenIssuanceID (Hash192), ShareMPTID (Hash192), Holder (AccountID), AssetScale (UInt8), MaximumAmount (UInt64), MPTAmount (UInt64), MPTokenMetadata (Blob).

3. Transaction models following existing patterns in the codebase (CommonFields with #[serde(flatten)], ValidateCurrencies derive macro, builder methods, flag enums with serde_repr).

4. Validation logic where appropriate: MPTokenIssuanceCreate validates transfer_fee <= 50000, MPTokenIssuanceSet rejects mutually exclusive TfMPTLock + TfMPTUnlock flags.

The UInt192 type name in definitions.json was renamed to Hash192 to match the rippled naming convention (fixed-length hashes use the Hash prefix, not UInt).

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

Before / After

Before:

• Binary codec has no awareness of Hash192 or any MPT fields/types
• No MPT transaction types in TransactionType enum
• No MPT ledger entry types
• Attempting to encode an MPT transaction would fail

After:

• Hash192 type handles 24-byte hashes correctly (fixed-length, no VL prefix)
• All four MPT transaction types serialize/deserialize correctly through both JSON (serde) and binary codec (encode/encode_for_signing/encode_for_multisigning)
• MPToken and MPTokenIssuance ledger objects deserialize from JSON
• 40 new tests across the transaction models, ledger objects, and binary codec

Test Plan

# Run all tests (40 new MPT-specific tests + existing suite):
cargo test --all-features

# Run only MPT-related tests:
cargo test mptoken --all-features
cargo test mpt --all-features
cargo test hash192 --all-features

# Verify no regressions:
cargo clippy --all-features -- -D warnings
cargo fmt --check

All pass with zero warnings and zero failures. The binary codec tests verify:

• Field ID encoding/decoding for all 7 MPT fields
• Transaction type code resolution (54-57)
• Ledger entry type code resolution (126-127)
• Field instance metadata (type, nth, serialized, signing, vl_encoded flags)
• Full encode() for all 4 transaction types with assertion on specific hex patterns
• encode_for_signing() prefix verification
• encode_for_multisigning() prefix and suffix verification
• Deterministic encoding (same input -> same output)