AWS Batch backend

I propose [AWS Batch](https://aws.amazon.com/batch/) as a new `clustermq` scheduler. Batch has become extremely popular, especially as traditional HPC is waning. I have a strong personal interest in making Batch integrate nicely with R (ref: https://github.com/wlandau/targets/issues/152, https://github.com/wlandau/tarchetypes/issues/8, https://wlandau.github.io/targets-manual/cloud.html).

Batch is super easy to set up through the AWS web console, and I think it would fit nicely into `future`'s ecosystem: maybe with something like `future::plan(future.aws.batch::future_aws_batch, template = "batch.tmpl")`, where `batch.tmpl` contains an [AWS API call](https://docs.aws.amazon.com/batch/latest/APIReference/API_SubmitJob.html) with the compute environment, job queue, job definition, and key pair. I think we could use [`curl`](https://github.com/jeroen/curl) directly instead of the much larger and rapidly developing [`paws`](https://github.com/paws-r/paws) package. The tricky part is how we retrieve the data back from an AWS Batch job. I'm not sure how to do that yet.