feat: implement custom PLug to forward headers using a Absinthe.Plug

[frfroes]

This PR closes #4

I implemented the solution described in the issue by @feliperenan where we have a custom Plug that takes the given headers from the conn and inject it into the Absinthe.Plug context in other to forward them to the downstream services on the resolver level.

[feliperenan]

If we proceed with this approach, I think it would be nice if we add how users of this library would plug this plug on their application. It could be here though, something like this:

@moduledoc """
omitted...

In order to use this plug, you need to plug this in your router.

  pipeline :my_pipeline do 
    plug Absinthe.Compose.Plug.ForwardHeader, ["authorization"]
  end
"""

[feliperenan]

Btw, I think would be better if we specify a key for the headers name, something like: plug Absinthe.Compose.Plug.ForwardHeader, headers: ["authorization"]

My main concern is if this plug starts to receive new arguments in the future (I don't have an example of that). If that happens, a new release of this library would result in a breaking change, requiring all users of this library to update their code before proceeding with the update.