Handle list with @uniqueItems trait in server code generation

[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.

[DavidOgunsAWS]

Closing as duplicate of #1670