Add support for omitting session token in canonical requests (#2473)

* Add support for omitting session token in canonical request

* Add tests to cover session token exclusion in signed headers

* Remove redundant session token insertion

* Drop mut canonical_headers

* Skip adding x-amz-security-token to signed headers if excluded

* 📎



* cargofmt

* Update changelog

* Update CHANGELOG.next.toml

Co-authored-by: John DiSanti <johndisanti@gmail.com>

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Co-authored-by: Russell Cohen <rcoh@amazon.com>
Co-authored-by: John DiSanti <johndisanti@gmail.com>