rbd: add GetSnapGroupNamespace() to get details about group snapshots

[nixpanic]

rbd_snap_get_group_namespace() can be used to get details about snapshots of an image, that were created as part of a group.

Checklist

• [x] Added tests for features and functional changes
• [x] Public functions and types are documented
• [x] Standard formatting is applied to Go code
• [x] Is this a new API? Added a new file that begins with //go:build ceph_preview
• [x] Ran make api-update to record new APIs

[nixpanic]

The reef CI jobs seem to have infrastructure issues :disappointed:

[phlogistonjohn]

The reef CI jobs seem to have infrastructure issues 😞

Yes. I've informed the ceph team in the #sepia slack channel. It affects a fairly wide variety of users (both the rpm-reef and debian-reef dirs got removed/renamed) and so I hope it gets resolved soon.

[phlogistonjohn]

@mergifyio rebase

[mergify[bot]]

rebase

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[mergify[bot]]

rebase

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[mergify[bot]]

rebase

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[mergify[bot]]

rebase

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[anoopcs9]

@Mergifyio rebase

[mergify[bot]]

rebase

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