rbd: VolumeGroupSnapshot support

[nixpanic]

Posting early for initial reviewing. Some parts can be split from this large PR into smaller pieces.

Notable changes:

• internal/rbd_types package with interfaces so that objects can be passed around cleaner
• internal/rbd/volume.go implementing the new Volume interface for rbdImage
• internal/rbd_group package for all RBD-group functionalities
• internal/rbd/group_controller.go for all CSI VolumeGroup service procedures

Commits in the current form will mostly change majorly. Reviewing per commit is not recommended, only check the files that you are interested in 😉

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