Terraform-provider-aws: AWS Lambda function invocation as data source (like CloudFormation custom resource)

Hiya,

CloudFormation has support for custom resources in templates - functionality that is a limited subset of what Terraform supports in the form of providers' data sources. One of the supported receivers for CFN custom resources is a Lambda function. When specified, CloudFormation _invokes_ the Lambda function and passes properties to the function as a request body. The response body is then available to the rest of the template as attributes.

It would be great to have similar functionality in Terraform. We can almost do this using the External Data Source escape hatch, by doing something like:

data "external" "example" {
  program = ["bash", "-c", "xargs -0 -I '{}' aws lambda invoke --function-name LAMBDA_ARN --payload '{}' /dev/stdout"]

  query = {
    id = "abc123"
  }
}

But this is fiddly to type, requires the AWS CLI to be installed and credentials are a bit of a pain to manage. It would be great if this had first-class support.

What do you think? If there's general approval of the idea - or it needs some tweaks - I'd be happy to submit a PR.