[FEATURE] Add support for PCS

[cartalla]

Add support for AWS Parallel Computing Service (PCS) which offers a managed Slurm cluster.

Add PcsConfig, similar to ParallelClusterConfig, and deploy/update the cluster using PCS instead of ParallelCluster.

[cartalla]

Right now PCS supports the AWS CLI, but doesn't support CloudFormation resources or CDK constructs so if I do this right now I'll have to use custom resources to deploy PCS constructs using the latest boto3 library.