[dagster-fivetran] Implement base poll method in FivetranClient

[maximearmstrong]

Summary & Motivation

This PR reworks legacy oll method and implements it in the FivetranClient:

• poll_sync is added based on legacy poll_sync
• the way of handling the connector sync status has been updated
  • Logic has been reworked and moved to FivetranConnector properties

Tests mock the request API calls and make sure that all calls are made.

How I Tested These Changes

Additional unit tests with BK