Refactor extension discovery and add base module for all extensions

[danschultzer]

Resolves #329

This may supersede #333

With this PR a base module is now expected in an extension that will clearly describe what the extension supports. This should prevent pretty much all Code.ensure_compiled?/1 calls, but is still backwards compatible.

For extensions missing the base module a warning will be printed with no MyCustomModule base module to check for ["Ecto", "Schema"] support found, please use Pow.Extension.Base to implement it).

A base module would look like this:

defmodule PowEmailConfirmation do
  @moduledoc false
  use Pow.Extension.Base

  @impl true
  def ecto_schema?(), do: true

  @impl true
  def phoenix_controller_callbacks?(), do: true

  @impl true
  def phoenix_router?(), do: true

  @impl true
  def phoenix_messages?(), do: true
end

[danschultzer]

As discussed in #333 (comment) I think it would be wise to require explicitly passing extensions, and maybe include the suggestion in #329 (comment) to have a test helper that can verify the extensions has been correctly set up.