feat(signature-v4-multi-region): add support for sigv4a package

[siddsriv]

Issue

Internal JS-5146

Description

Add support in signature-v4-multi-region for the @smithy/signature-v4a package. Partner PR: https://github.com/smithy-lang/smithy-typescript/pull/1319

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[Lemour-sudo]

Do we have any idea when this can be merged?

[github-actions[bot]]

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