The details of the newly added sources. This is a convenience method that creates an instance of the
List.Builder avoiding the need to create one manually via List#builder().
When the Consumer completes, List.Builder#build() is called immediately and its
result is passed to #sources(List).
Parameters:
sources - a consumer that will call methods on List.Builder
Returns:
Returns a reference to this object so that method calls can be chained together.