Add sparse when boxed

[sugmanue]

Background

When a Smithy 1.0 is converted to 2.0, then for lists and maps if the target value has a @box trait the corresponding collection will be marked as @sparse. This should properly implement in 2.0 the implicit idea that the value of the collection can be null.

Testing

• Added unit tests.

Links

• Links to additional context, if necessary
• Issue #, if applicable (see here for a list of keywords to use for linking issues)

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

This pull request does not contain a staged changelog entry. To create one, use the ./.changes/new-change command. For example:

./.changes/new-change --pull-requests "#2930" --type feature --description "Add sparse when boxed"

Make sure that the description is appropriate for a changelog entry and that the proper feature type is used. See ./.changes/README or run ./.changes/new-change -h for more information.