migrating api/auth/aws from aws-sdk-go to aws-sdk-go-v2

[rahulkaukuntla]

Description

What does this PR do?

aws-sdk-go is being deprecated by July 31, 2025, and any usages of this dependency need to be migrated to aws-sdk-go-v2. It seems that api/auth/aws depends on this old dependency, which means that importing it, along with the vault repo in general, causes both versions of the sdk to be imported, blowing up the size of any binary that depends on api/auth/aws. To remediate this issue, I have:

1. Replaced deprecated AWS SDK v1-style imports (session, ec2metadata, etc.) with modern AWS SDK v2 modules like config and imds.
2. Removed the go-secure-stdlib/awsutil dependency (which itself depends on the v1 SDK) in favor of using standard AWS SDK v2 interfaces (aws.CredentialsProvider).
3. Updated the EC2 metadata handling to use imds.GetDynamicData(ctx, &Input) and properly read from io.ReadCloser responses.
4. Switched IAM auth logic to retrieve credentials from cfg.Credentials via Retrieve(ctx) and manually build the loginData map.

I have verified that this fix works by locally importing this updated library into a local instance of the datadog-secret-backend executable, and using that, I was successfully able to pull secrets from my AWS-enabled Hashicorp Vault. The generated datadog-secret-backend executable decreased in size from ~16 MiB to ~11 MiB, since we don't have to ship aws-sdk-go anymore, showing just how important this change is to us.

If this PR is approved, I can officially bump the version in the datadog-secret-backend repository.

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
  • LTS: If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.
• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've documented the impact of any changes to security controls.

Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.