Add XLS-89 parser

[Patel-Raj11]

This PR has following changes:

1. Add xls89 parser to parse and validate hex encoded MPTokenMetadata object to JavaScript object. The parser converts compact keys to long keys along the way to enable consumers of this package to readily consume readable long keys. Ongoing PR to add support for compact keys.
2. Update xls26 parsers to parse MPT stanzas specifically to support trust_level overrides for MPTs.
3. Add some unit tests.