[chronik-client] Implement keepAlive to preserve longlived connections

Summary:
Left to its own devices, websockets will disconnect in ~45s if no information is being exchanged between client and server.

Standard practice is to send ping msgs from the client, which the server responds to with pong. These methods are usually standardized.

Such methods are already implemented in chronik-client both in nng and in-node. It is possible to manually implement a ping-pong keepAlive at the app level, instead of in this library. However, it makes sense to do it in chronik-client, which already has a websocket manager that performs other typical websocket mgmt tasks like autoReconnect. It's also easiest to clearInterval on the ping method inside the existing websocket manager, which is already handling onopen and onclose methods.

Traditional TCP keepalive is not practical for websocket connections:
https://stackoverflow.com/questions/23238319/websockets-ping-pong-why-not-tcp-keepalive
https://websockets.readthedocs.io/en/stable/topics/timeouts.html

A ping interval of 30s was selected based on testing connections with NNG chronik websocket. Running for 5 min, a ping interval of 30s showed no disconnects (vs no ping interval would show 3-5). Disconnects were observed with a ping interval of 45s.

This diff does not prevent disconnects which may still happen for a variety of network reasons. App developers are expected to handle the disconnect event and recovery methods to catch missed events.

Test Plan: npm test, tests pass, terminal does not hang (confirming the interval is cleared by ws.close())

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14831