[ChronikClient] Add "UNKNOWN" protocol type

Summary:
In case we add a new token protocol in the future, this gracefully allows querying the tx without throwing an error.

E.g., if we were to add BLP (Better Ledger Protocol), and were to connect an old ChronikClient version to a Chronik instance that implements BLP, this library (without this diff) would always throw an error for any transactions containing BLP tokens. However, it is better to return that the token protocol is unknown.

A corollary is that app devs are now forced to handle UNKNOWN token protocols, however, this is something that should be handled gracefully. Right now, it's impossible to handle it gracefully.

Test Plan: npm test && npm run integration-tests

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D18155