Implement Custom Middleware/Authorization for import endpoint

[tylersamples]

The importer config should support a middleware option where a it accepts a capture Middleware.import/0.

A high level middleware should look like so:

defmodule Middleware do
  use Tesla

  plug Tesla.Middleware.BaseUrl, "https://api.excourtbot.com"
  plug Tesla.Middleware.Headers, [{"authorization", "token xyz"}]
  plug Tesla.Middleware.JSON

  def import() do
    get("/todays-data")
  end
end