[ecash-herald] Parse SwAP txs

Summary:
T3060

Parse SWaP txs

Handles some observed inconsistencies between spec and recent network txs (active apps appear to use ASCII for encoding / decoding sats). Stub parsing for uncommon methods, can be expanded as these txs are observed.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14054