refactor(sdk): Support multiple contracts in ChainEventPoller

[teogeb]

Refactored ChainEventPoller to support multiple contracts simultaneously. Now the class is a singleton and we can define contract methods as a listener parameter. This optimizes log fetching by reducing required HTTP calls to the JSON RPC as there needs to be only one poller running even if multiple contract events are listened.

Also we don't now need to create a Contract instance just for adding event a listener. This allows us to simplify e.g. Operator class (in a follow-up PR).

[harbu]

To address your point

Added try-catch around the event listener so that the polling doesn't stop if a listener throws an error.

We had a discussion on video meeting (me and Teo ofc), maybe better to leave out and have it as caller's responsibility (especially since it was like this before).

On that front, regarding the async case, maybe eslint rule that giving async callback to place where (sync) callback is expected would be an error?