Replace `once_cell` with `std::sync::LazyLock` in `aws-sigv4` (#4050)

## Motivation and Context
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Replaces an external dependency with functionality provided by `std`

## Description
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## Testing
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## Checklist
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- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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Co-authored-by: Landon James <lnj@amazon.com>