[chronik] Electrum: Implement the blockchain.header.get endpoint

Summary:
https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html#blockchain.header.get

The boring part is to get the errors to align with Fulcrum.

Depends on D18167.

Test Plan:

./test/functional/test_runner.py chronik_electrum_blockchain

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18169