chore(lib-dynamodb): reduce object copying in iterators

[kuhe]

Issue

#4151
paired with smithy-lang/smithy-typescript#638

• reduce object copying in iterators

This reducer style makes pointless object copies via { ...acc}:

.reduce((acc, [key, val]) => ({ ...acc, [key]: val }), {});

We know that removing these copies will have no side-effects because the initial accumulator value is {}, and not an external object that should not be mutated.

New pattern:

.reduce((acc, [key, val]) => (acc[key] = val, acc), {});

[kuhe]

split codegen changes into #4198

[github-actions]

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