Add support for vpc-lattice auth

[sseelbach-27]

Description

Adds support for vpc-lattice authentication according to https://docs.aws.amazon.com/vpc-lattice/latest/ug/sigv4-authenticated-requests.html

[FantasticFiasco]

Hi @sseelbach-27 and thanks for the pull request!

I'm currently engaged with another pull request at the moment and will start with this one after the other one is merged. This make take a week, and doesn't indicate that I don't value your contribution. I truly do, it's just that my time is sparse. I hope you understand.

[FantasticFiasco]

This will require some analysis since Sigv4a might mean other changes as well, something the library currently has no support for at all.

[macmanad]

Hi, just wanting to give this PR a bump and ask if there's a chance of it moving soonish?

[FantasticFiasco]

The major parts of a functionality like this is to understand the details and provide test coverage for the feature. Unfortunately, the first part is impossible to cover in a PR and the second part is lacking, i.e. there is a greater investment from my part which lowers its priority.

[FantasticFiasco]

There is a pre-release published on nuget. Can you please verify that it works as intended?