[chronik-client] Default true for websocket keepAlive

Summary:
Plan on using this on all apps that I use chronik websockets with. It would be weirder to not want to use it.

So, the default should be true.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14852