bufferstreams: Add basic implementations of core BS traits

For BufferSubscriptions, this change provides a generic implenmentation
that tracks requests and cancellations.

For BufferPublishers and BufferSubscribers, we provide test
implementations that let a user manually control the flow of events
between the two objects.

The traits themselves have also been updated to be more generic--instead
of forcing Arc/Weak pointers for shared objects, we now use generic
owned types for a BufferPublisher's BufferSubscriber and a
BufferSubscriber's BufferSubscription.

To make it possible to hold into a handle to a BufferSubscriber while a
BufferPublisher owns it, we provide a generic implementation of
BufferSubscriber for any Arc<BS: BufferSubscriber> that delegates to the
underlying subscriber.

Bug: 296449936, 296100790
Test: atest libbufferstreams-internal_test
Change-Id: Ibbf925d2dfb85f606baa3dc1f9722440af4f862c