Allow configure repo_opts per query

Now we could only configure `prefix` in `repo_opts`, is it possible to make the option also available in `Dataloader.Ecto.load`? For example
```elixir
Dataloader.Ecto.load_many(Repo, :assoc_key, record,
    repo_opts: [prefix: "my_prefix"], other_option: true)
```

Now ecto-sql 3.0 already supported `prefix` well, hope dataloader could do better.