Update uses of SetShape to ListShape

[Velfi]

Also, remove the old Set-handling logic. Sets no longer exist in Smithy IDL v2.0.

[DavidOgunsAWS]

Smithy 2.0 introduced the uniqueItems constraint trait for lists and is the recommended type + constraint to replace (now removed from IDL) SetShape. This constraint trait is only expected to be handled with server side request validation, but clients should not care.

Further notes:

• uniqueItems trait requires the List member type have specific value equality semantics which should align fairly well with Rust types and traits.
• SetShape class still exists for 1.0 model processing compatibility. SetShape is a subclass of ListShape so handling of ListShape with the uniqueItems trait should (in many cases) satisfy Set handling as well in 1.0 models unless the prior implementation of Sets had further semantics that do not apply to List/uniqueItems.

[david-perez]

As of #2232, @uniqueItems is honored by server SDKs.

Keeping this open to track modifying all uses of SetShape in smithy-rs to ListShape or CollectionShape; since Smithy parses v1 models by transpiling set shapes into ListShapes, we should not have to use SetShape anywhere. I've modified the title too.