Allow setting custom callbackExecutor in RequestBuilder

This is more of a feature request than a bug.

I'm wondering if there's any particular drawbacks of exposing the package private method `into(@NonNull Y target, @Nullable RequestListener targetListener, Executor callbackExecutor)` from [RequestBuilder#L618](https://github.com/bumptech/glide/blob/master/library/src/main/java/com/bumptech/glide/RequestBuilder.java#L618). From a cursory glance, it doesn't appear that would introduce new race conditions as `submit(width, height)` already does this with the direct executor.

I am loading a bitmap asynchronously to perform some additional operations on it and not into a view. The current setup would require two thread hops, one by Glide to the main thread to perform the callback, and another by me to a background thread to continue processing the bitmap. I would like to use another thread pool backed executor to perform the callback for performance reasons since the UI thread may be busy at the time when the request is complete.