Add volume module type for persistent volumes

[eysi09]

Add a volume module type that allows users to use persistent volumes. Other modules types, e.g. the container module type, can reference the persistent volume and multiple modules will be able to share a persistent volume.

In terms of implementation, we'll create a base-volume module type which abstracts away the most common features that are likely to be shared by multiple providers. We'll extend that with a persistent-volume-claim module type that can be used specifically by the kubernetes and local-kubernetes providers, and that has the same spec as the PersistentVolumeClaim resource.

In the future, we could extend the base-volume for other providers as well.