Re: [streams] Move BYOBRequest after BYOBController and add FS example (b430f33)

We need to provide a concrete example of how to implement such a controller representing a passive data source and reacting to both BYOB and default read adaptively.

The controller can know that there's a BYOB reader and it's receiving a BYOB read request by looking at `controller.byobRequest`, but there's no such API for checking read requests from a default reader.

I think HWM needs to be set to 0 (we've changed the default from 1 to use 0 for BYOB controller at some point. Maybe for the same reason? I don't remember...) to expose the default-read signal (pull is invoked only when there's at least one default-read request).

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