[ecash-herald] Add new script to support testing, sendMsgByBlock

Summary:
T2972

It's impractical to keep adding more and more test blocks as the app covers more and more use cases. Often when adding a new feature, you just need to check the result one a single block, and not all of the test blocks.

This test script allows you to generate a msg and send to your test channel by blockheight, e.g. npm run sendMsgByBlock 700000

Test Plan:
npm test
npm run sendMsgByBlock 700000 and msgs send

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D14036