Feature Request: Switch to Tesla for HTTP library

[OldhamMade]

If Contentful.ex switched to Tesla, this would enable users to choose which underlying HTTP library to use by defining an Adapter. For example, a team could choose to use Finch (instead of the current HTTPoison) because it is already in use and they want to ensure consistency.

Tesla would also allow teams to include middleware, such as tesla_statsd, to help monitor their request volume.