[electrum] add a script to automate updating of checkpoints

Summary:
Add a patch recipe to update electrum checkpoints, similar to the script automating chainparams or seeds.

The get_merkle_root script called without params will fetch a checkpoint 50 blocks before the current tip of the specified electrum server.

Test Plan:
Check that this creates the expected commit

cd ./contrib/source-control-tools
./patch-recipes/update-electrum-checkpoints.sh

Run Electrum ABC, check that it connect succesfully to all 3 servers (tip >= 50 + checkpoint_height)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15343