[herald] Print ascii string for unknown miners with well-pushed ascii in coinbase

Summary:
Parse new miner(s) first observed at block 825737.

This could be a new pool, or a new miner, or simply a pattern that happens to fit some recently observed blocks. Either way, it is an improvement on calling them all 'unknown'.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15083