feat: add test coverage to restxml

[dayaffe]

Describe the feature

In tests for restxml/xml-enums.smithy we currently tests structures, lists, nested lists, and enum lists, but do not tests sets and maps. The following comment was left in code found here:

//  Todo: sets, maps
//  1.  This will be part of: XmlEnumsInputOutput
//    fooEnumSet: FooEnumSet,
//    fooEnumMap: FooEnumMap,
//
//  2.  Then add this:
//    set FooEnumSet {
//        member: FooEnum,
//    }
//
//    map FooEnumMap {
//        key: String,
//        value: FooEnum,
//    }
//

Use Case

We should support sets and maps in smithy files sent over restxml protocol and therefore should also provide adequate test coverage of these scenarios.

Proposed Solution

Add test coverage and address any gaps in our implementation.

Other Information

Related issue: #710

Acknowledgements

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

[jbelkins]

Is it possible that Smithy has added these as protocol tests in the restxml suite since the TODO was written?
(Such a test is not Swift-specific and I would expect it to be in the cross-platform protocol test suite if it is needed to ensure an implementation understands the protocol)

[sichanyoo]

This ticket will be part of clean-up: #1217.
Look into whether the files in same directory are covered by protocl tests in Smithy project already: https://github.com/smithy-lang/smithy/tree/main/smithy-aws-protocol-tests/model/restXml