[electrum] move checkpoint data to a json file

Summary:
And add an option to the get_merkle_root script to output the file in the correct format.

This will make it easier to automate the checkpoint updating without using hacky way to modify the networks.py source file.

Depends on D14981

Test Plan:
The two json files in this diff were generated via:

scripts/get_merkle_root 822000 --json-output electrumabc/checkpoint.json
scripts/get_merkle_root 1477500 --json-output electrumabc/checkpoint_testnet.json  --server telectrum.bitcoinabc.org:60002:s

Run the software, check that we are connected, that the servers are not blacklisted (would happen if the merkle root was wrong).

Build some release binaries and test them, to make sure the new data files are properly packaged.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14985