[chronik] Electrum: Implement the blockchain.estimatefee endpoint

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

This requires a bridge for a better estimate if the mempool is full.

Depends on D18161.

Test Plan:

./test/functional/test_runner.py chronik_electrum_blockchain

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18167