[ecash-herald] Show sent amount for Cashtab msgs

Summary:
T3200

Show xec send info for Cashtab msg txs

Test Plan: npm test, image output for Cashtab msg shown

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14226