Doesn't work with federated accounts

[Speculor]

Using a federated (ie SSO) account does not work with the existing aws_mixin.rb. It throws the following:
Must specify userName when calling with non-User credentials (Aws::IAM::Errors::ValidationError)
To fix I had to update the get_aws_account_id method at line 34 to use the STS client instead of the IAM client:

def get_aws_account_id(credentials:)
    iam = Aws::STS::Client.new(region: 'us-east-1', credentials: credentials)
    user = iam.get_caller_identity
    user[:arn].match('^arn:aws:sts::([0-9]{12}):.*$')[1]
  end
end

[ecout]

Did you submit a pull request? Thanks!