[ecash-tg-bot] Show blockheight and tx count in block update msgs

Summary:
T2972

Add a call to chronik.block(blockHash) to get more information about the block, instead of the blockhash.

Test Plan: Add your bot and channel info to secrets.js, then node.js. Confirm msg format when a block is found.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D13237