Switch to Tesla as HTTP client

[alexgleason]

In the spirit of adapter-based clients, it would be good if the http client were configurable, too.

Tesla is the best Elixir HTTP client because it's all of them and none of them.

[swelham]

I agree that having the http client configurable would be a great enhancement. However, I would first look to consider an approach that doesn't require a new dependency.

Open to any suggestions on this.