Simplify creation of client

[ssoper]

Currently the client setup is kinda complex as it was intended to abstract away all the hard parts of managing multiple broker accounts. But each broker account should be configurable on its own and with Batil only supporting one broker at this time (E*TRADE) it doesn’t make sense to have all this abstraction, at least not yet.

Follow the Alpaca model where you can set everything up with a single line of code and all the endpoint specific calls are held by the client vs. having them passed in. Likely this will require re-thinking how to do testing (DI).