Standardize C interface for stream exchange

Following up on #57 where we figured out the correct stream exchange and synchronization semantics and a Python interface for doing so, we need to do the same for a C interface.

TLDR from #57:
- Frameworks aren't necessarily equipped to share streams with lifetime controls outside of their own internal execution, so adding a stream member to the dlpack struct won't suffice
- Consumer can optionally provide a stream to the producer when asking for the dlpack object
- Producer is responsible for guaranteeing that the data in the dlpack object they produce is safe to operate on in the stream provided by the Consumer

cc @tqchen @harrism @jrhemstad @rgommers @leofang @oleksandr-pavlyk @szha @veritas9872