add `computeTxId` as a utility function

[gudnuf]

Description

Discussion of this here, and related CDK PR

I am opening this PR because I want to ability to create an identifier for a "transaction" that can be embedded in a URL rather than an entire token. We will then store tokens locked to a pubkey in our database and map this txid to that token.

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Changes

Adds a utility function computeTxId that accepts `Array and

• orders all Y's in descending order
• concatenate them
• sha256 hash

PR Tasks

• [x] Open PR
• [x] run npm run test --> no failing unit tests
• [X] run npm run format