Add support for signed GET requests for aws authentication

[bluestealth]

Adds support for signed GET requests of GetCallerIdentity for authenticating to vault using the aws iam method.

This is related to aws/aws-sdk-go-v2#1137

[GromNaN]

👍🏻 Thank you for working on it. Supporting presigned GET requests is a valuable feature to make integration easier. AWS SDKs offers method to presign requests, but we have to use "internals" in order to catch method/path/headers/body of the sts request.

[GromNaN]

Hello, is there any security concern before adding this feature?

[maxcoulombe]

Hey @bluestealth, thanks a lot for your work on this. Kudos for thinking about the tests and docs as well.

The code looks sound, my only 2 comments are really minor nitpicking. Do you think it'd be possible to update the branch to resolve the conflict and add a changelog entry so I can have one last look that everything is in order before approving?

The changelog could be something like:

10961.txt

```release-note:improvement
auth/aws: Added support for signed GET requests for authenticating to vault using the aws iam method.

[maxcoulombe]

Looks good, thanks for the modification & adding the test case.

Don't worry about the Vercel step, from what I understand it does not work with forks. I validated the doc changes with cd website && make website and it's ok.

[maxcoulombe]

This completely slipped under my radar, apologies for the delay and thanks for that contribution!