[chronik] Add the blockchain.scripthash.subscribe endpoint

Summary:
Compute the status and notify to subscribers: https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html#blockchain-scripthash-subscribe

Depends on D18108.

Test Plan:
Tested against fulcrum and an address of mine to check the format matches (status expected)

./test/functional/test_runner.py chronik_electrum_*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18087