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)

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[github-actions]

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./.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.

[sugmanue]

We will take a different approach to solve this to avoid the risk of breaking existing models.