fix(core/protocols): performance improvements for shape serde traversal

[kuhe]

Issue

#7520

Description

This optimizes the traversal of shapes that contain nesting unions where most keys are empty.
This mostly applies to DynamoDB items having the AttributeValue type.

Testing

Added unit tests, manual testing shows speed improvement.

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts).
• If you wrote E2E tests, are they resilient to concurrent I/O?
• If adding new public functions, did you add the @public tag and enable doc generation on the package?