Move event streaming code to smithy-swift

[jbelkins]

Describe the feature

Some event streaming code (i.e. for marshalling & unmarshalling event stream types) is located in aws-sdk-swift, even though event streaming is a core Smithy feature.

Locate non-AWS specific event streaming code that is in aws-sdk-swift & move it to smithy-swift. Also, add an event streaming operation to the Test SDK to verify that smithy-swift can build an SDK with event streams.

Use Case

To support core Smithy features in smithy-swift and properly modularize the SDK

Proposed Solution

Move types & break dependencies as needed.

Other Information

No response

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

[jbelkins]

Event streaming code has been modularized per standard ref. architecture. This included moving code out of SDK and into modules provided by smithy-swift.

Closing since this is already tracked by #1384 and #1385.