Feature request: Add Source#blockingQueue() operator for easy integrating with non reactive world?

Motivation:
Currently, I have to use the `unfoldReosurce()` to integrate with a blocking queue. I think it would be nice to have this out of the box.

Start by reading the existing unfoldReosurce() approach and the Source API context for the proposed blockingQueue() operator. Determine the expected blocking-queue integration and operator semantics from the surrounding stream APIs; done means a documented, tested out-of-the-box operator is available without requiring unfoldReosurce().

Written by the indexing model from the issue text.