[chronik] Add a subscription to txid

Summary:
So we can monitor the status change for a single txid. Typically:
subscribe, broadcast, check it enters the mempool, check it finalizes.

Test Plan:

./test/functional/test_runner.py chronik_ws

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18230