Centralize event streams

[JordonPhillips]

To summarize what this PR does:

• smithy-event-stream package was removed, and its contents were moved into core.
• The event stream classes were changed from protocols to concrete classes. The expectation is that protocols implement the publisher and receiver, then those are combined into one of the three stream types.
• Methods were added to the ClientProtocol interface to create the publishers and receivers.
• code gen was updated to use the new types

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