UX improvement: calls on callbacks

The `add` method returns a `BaseResponse`, on which you can find calls using `calls`.

The `add_callback` method doesn't. Users can get calls from the generic `RequestsMock` object, but that requires searching for the right request, for which `responses` provides no native methods.

Solutions:

- Let `add_callback` return `BaseResponse` (probably not possible)
- Add built-in way to search for calls on the `RequestsMock` object, preferably those belonging to a specific response