[chronik] Add block subscription functionality to websocket.rs within bitcoinsuite-chronik-client

Summary:
The code adds WebSocket functionality for a Bitcoin blockchain explorer (Chronik), specifically the subscription and unsubscription to blocks

• Creates a WebSocket connection to Chronik
• Subscribes to block updates with ws_endpoint.subscribe_to_blocks()
• Verifies subscription status with assert_eq!(ws_endpoint.subs.blocks, true)
• Tests unsubscribing with ws_endpoint.unsubscribe_from_blocks()
• Verifies unsubscription worked with assert_eq!(ws_endpoint.subs.blocks, false)
• Resubscribes again

Test Plan:
Please set BUILD_DIR env to export BUILD_DIR="/path/to/build_dir
UNIX: ./contrib/teamcity/build-configurations.py build-bitcoinsuite-chronik-client

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: tobias_ruck, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17936