feat(cloudfront-signer): enable SHA256 for cloudfront-signer

[kuhe]

Issue

https://aws.amazon.com/about-aws/whats-new/2026/04/amazon-cloudfront-sha-256-signed-urls/

Description

Updates @aws-sdk/cloudfront-signer to enable use of the new option introduced by Amazon CloudFront.

Testing

added unit, integ, e2e tests

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts) or E2E tests.
  • It's not a feature.
• My E2E tests are resilient to concurrent i/o.
  • I didn't write any E2E tests.
• I added access level annotations e.g. @public, @internal tags and enabled doc generation on the package. Remember that access level annotations go below the description, not above.
  • I didn't add any public functions.
• Streams - how do they work?? My WebStream readers/locks are properly lifecycled. Node.js stream backpressure is handled. Error handling.
  • No streams here.

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