[Chronik] Require subscribing to WebSocket block updates

Summary:
Require sending WsSub protobuf message to the server to subscribe to blocks, before sending out block updates.

NNG Chronik sent these unconditionally, but since we now have a more flexible subscription API, we can give users different subscription options.

Test Plan: ninja && ./test/functional/test_runner.py chronik_ws

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13787