Add support for Ecto parallel preloads

Replacing use Ecto.Repo with use Appsignal.Ecto.Repo allows the Ecto instrumentation to keep context across the Elixir processes spawned by Ecto preload queries, ensuring that these queries are correctly instrumented:

defmodule MyApp.Repo do
  # replace `use Ecto.Repo` with `use Appsignal.Ecto.Repo`
  use Appsignal.Ecto.Repo,
    otp_app: :my_app,
    adapter: Ecto.Adapters.Postgres
end

See the changelog for AppSignal for Elixir package 2.8.2 for more information.