Add Alpaca API v2 support, update Polygon for v2 aggregate support

This release introduces the ability to configure the version of the Alpaca API to target. This is currently primarily intended for users using Alpaca power accounts. It also includes a new historic_agg_v2() method for Polygon's API. The V2 endpoint reportedly does not experience some of the performance issues users have noticed with the V1 endpoint, so if your code has been having trouble with that, please try the new endpoint.