Every “noun” should have a message, so Volume should be a message

[saad-ali]

Recommendation from a gRPC author after review of CSI API:

Every “noun” in the API should have a proto message associated with it.

Since, we have operations like CreateVolume(...), ...PublishVolume(...). We should have be a message defining the Volume` object.

We already have something like that, VolumeInfo, so maybe consider renaming that to Volume.

[saad-ali]

Closed with #182