[ecash-agora] Add subscribeWs and unsubscribeWs

Summary: These take a WsEndpoint object from ChronikClient.ws, and subscribe to / unsubscribe from the requested type of updates from the Agora plugin.

Test Plan: npm run integration-tests

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16845