default master key provider value needs to be namespace-specific

[mattsb42-aws]

Problem

If the caller does not specify a master key provider, we assume that they want to use the AWS KMS master key provider. However, currently the entry point ID references is the generic aws-kms. This could create unexpected behavior if another module is installed on the caller's system that also registers an entry point with that name.

Solution

Make the default value the namespace-specific value aws-encryption-sdk-cli::aws-kms.