Make :goth option optional

[Wojtek Mach]

A user may already have a token from another place. Here's one easy way to grab it:

$ gcloud auth application-default print-access-token
ya29.a0...

If a user already has access to a token, it is unnecessary for them to go through Goth, they can set the auth: {:bearer, token} on their req instead. For us, this can remove a ton of boilerplate from our unit tests. We should have one unit test where we do use Goth though. In integration tests we should use it too.

[Alexandre de Souza]

What if we use this function so they could start Req with auth already set?

  defp auth(%{auth: auth}), do: auth
  defp auth(%{goth: goth}), do: {:bearer, Goth.fetch!(goth).token}

[José Valim]

I wouldn't worry about this. If someone has this use case, then they can open an issue/PR to better discuss it.