Support for AWS ECS Backend

Requesting support for AWS ECS with Fargate support. A previous issue was opened with this same request, but closed in favor of Dask CloudProvider. I require Dask Gateway so admins can ultimately control allocation of resource upon request from users, but we like to run everything on ECS with Fargate. ECS support would really streamline the deployment of Dask Gateway for us, as it would not introduce maintenance of a Kubernetes/Hadoop cluster into the mix while still letting admins keep control of resource allocation.