[chronik] Electrum: Implement the *.get_first_use endpoints

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

Note that fulcrum returns a duplicated field for this request (height and block_height) but we only follow the specification.

Depends on D18153.

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/D18154