fix: correctly codegen defaults for enum shapes

[ianbotsf]

Issue \

(none)

Description of changes

Enum shapes don't have correct defaults codegenned for them. For example, in a structure's builder class:

public var mode: aws.sdk.kotlin.services.neptunedata.model.GraphSummaryType? = "basic" // <-- compiler error

This change adds handling of defaults so that they're resolved correctly from the enum's members.

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

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