[chronik-client] Organize code to prepare for additional in-node class

Summary: Move ChronikClient to its own file in preparation for a new class specific to in-node chronik client connections. index.ts remains as entry point and where all classes available to installers of the chronik-client module are located.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14823