[electrum] add a script to get the merkle root for a block height

Summary:
Connect to a fulcrum server and get the block verification merkle root for a specified block height, or by default for a recent block with enough confirmations for safety against potential block parking by Avalanche.

This makes updating the checkpoints (D14972) easier and is a first step for automating the update.

Test Plan:

scripts/get_merkle_root -h
scripts/get_merkle_root
scripts/get_merkle_root -v
scripts/get_merkle_root -v 822000
scripts/get_merkle_root 822000
scripts/get_merkle_root -s fulcrum.pepipierre.fr:50002:s -v

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14981