feature: add managed default VolumeSnapshotClass for Cinder

[hebelsan]

How to categorize this PR?

/area quality
/kind enhancement
/platform openstack

What this PR does / why we need it:
This PR adds managed default VolumeSnapshotClass for Cinder.

Which issue(s) this PR fixes:
Fixes #1318

Special notes for your reviewer:

Release note:

Add managed default VolumeSnapshotClass for Cinder

[gardener-prow]

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[github-actions]

This change introduces volume snapshot functionality for OpenStack Cinder storage by adding a default VolumeSnapshotClass resource. The addition enables users to create and manage volume snapshots in Kubernetes clusters using the CSI Cinder driver, with configurable default behavior through a new values parameter.

Walkthrough

• New Feature: Added VolumeSnapshotClass template for CSI Cinder driver enabling volume snapshot capabilities in Kubernetes clusters
• New Feature: Introduced managedDefaultVolumeSnapshotClass configuration option allowing users to control whether the snapshot class is set as default
• Chore: Enhanced storage configuration with snapshot management capabilities for OpenStack environments

_{Model: claude-sonnet-4-20250514 | Prompt Tokens: 455 | Completion Tokens: 151}