Add config for sagemaker_runtime_a2i

[Gazler]

To allow ex_aws to be used with the Amazon Augmented AI API (endpoint a2i-runtime.sagemaker.REGION.amazonaws.com) a new service map override has been added named :sagemaker_runtime_a2i to reflect the existing :sagemaker_runtime

https://docs.aws.amazon.com/augmented-ai/2019-11-07/APIReference/API_StartHumanLoop.html

Example:

operation =
  %ExAws.Operation.JSON{
    http_method: :get,
    path: "/human-loops",
    service: :sagemaker_runtime_a2i
}

ExAws.request(operation)

[bernardd]

Thanks @Gazler!