Add MPTokenMetadata encoding, decoding, and validation utilities

Description

This PR implements complete support for MPToken metadata handling according to the XLS-89 standard, including encoding, decoding, validation, and comprehensive test coverage for all functionality.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where needed
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes

Changes

• Added ParsedMPTokenMetadata and ParsedMPTokenMetadataURI structs with compact JSON tags
• Implemented EncodeMPTokenMetadata - encodes struct to hex with compact keys and sorted fields
• Implemented DecodeMPTokenMetadata - decodes hex to struct, handling long and compact field names
• Implemented ValidateMPTokenMetadata - validates hex against XLS-89 rules (required fields, formats, sizes, collisions)

Notes (optional)

• XLS-89 Standard: XRPL XLS-89d Specification
• Reference Implementation: xrpl.js MPTokenMetadata utilities