[chronik] Electrum: implement the blockchain.relayfee endpoint

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

Depends on D18186.

Test Plan:

./test/functional/test_runner.py chronik_electrum_blockchain

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18187