feat(signature-v4a): create SignatureV4a JavaScript implementation

[kuhe]

moved from https://github.com/smithy-lang/smithy-typescript/pull/1250 so I have write access.

The goal of the original PR is to have a sigv4a implementation without the CRT, so it can be used in browsers.

[kuhe]

• [ ] tag https://github.com/bmaggia as author in the squash commit before merging

[zeyad001]

Looking forward to this functionality