Handle streaming data

[jeffhandley]

Broken out as a separate issue from #5719. Per #5719 (comment):

• The best current proposal seems to be adding another type StreamingDataContent which does not inherit from DataContent but rather inherits AIContent directly. The person instantiating this type would need to pass in a suitable factory/callback to the constructor that would return the stream. The person consuming this type would call content.TryTakeStream(out var stream), which may or may not give them a stream (depending on whether it's capable of supplying more than one reader) and if it does, hands responsibility for disposing that stream instance to the consumer.
• I'm unclear on how this interacts with JSON serialization. Is there at least one practical case where this actually avoids buffering a large file in memory? I know it probably doesn't help with receiving any data from JSON-over-HTTP. But maybe it does help when sending JSON-over-HTTP. If there are no cases today where this actually avoids buffering in practice, then we might choose not to prioritize doing this now.