WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
There are objective metrics around performance types - the user can request them, and has to make them a hard limit (wait until this can be satisfied) or a soft limit (give me something close).  This is a characteristic that pods already have, so it's better to frame this in terms of the resource proposal than to invent something else.