[ecash-herald] Preliminary parsing of empp and slp v2 txs

Summary:
T2972

Add stub functions to parse empp pushes (for slp v2 only) and limited info about slp v2 txs

This prevents garbled parsing of slp v2 txs, but does not give a perfect description of the txs. For a Telegram bot, this type of description should come from a parsing library or an indexer.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14072