Refactor aws-sigv4 for Event Stream (#648)

* Remove serde dependency from aws-sigv4

* Clean up aws-sigv4 `sign` module

* Move HTTP-specific sigv4 code into its own module

* Clean up `http_request` module

* Eliminate serde dependency

* Move bytes to dev-dependencies

* Fix aws-sig-auth build

* Add Event Stream signing

* Update changelog

* Return signature from signing functions and add it to property bag in requests

* CR feedback

* Further simplify parameter sorting