Add snapshot limit enforcement for WCP with per-volume serialization

[deepakkinni]

What this PR does / why we need it:
This PR implements snapshot limit enforcement for WCP environments to prevent users from exceeding a configurable maximum number of snapshots per volume.

What's Changed:

• Added support for reading snapshot limit from namespace annotation csi.vsphere.max-snapshots-per-volume
• Default limit is 4 snapshots per volume, with a hard cap of 32
• Implemented per-volume mutex to serialize snapshot creation and prevent race conditions
• Feature is gated behind the snapshot-limit-wcp FSS flag
• Returns proper error when limit is reached, including current snapshot count

How it Works:
When creating a snapshot, the driver queries existing snapshots for the volume and checks against the configured limit. If the limit would be exceeded, the request is rejected with a FailedPrecondition error. Per-volume locking ensures that concurrent snapshot requests are serialized to prevent race conditions where multiple requests could bypass the limit check.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Testing done:
Unit tests.

Unit Tests Added

TestGetSnapshotLimitFromNamespace - Tests for snapshot limit configuration:

• WhenAnnotationExists_ValidValue - Reads valid limit from namespace annotation
• WhenAnnotationExists_ValueEqualsMax - Handles limit set to maximum (32)
• WhenAnnotationExists_ValueExceedsMax - Caps values exceeding 32 with warning
• WhenAnnotationExists_ValueIsZero - Allows zero limit (disabled)
• WhenAnnotationExists_ValueIsNegative - Rejects negative values
• WhenAnnotationExists_InvalidFormat - Handles non-numeric annotation values
• WhenAnnotationMissing - Uses default limit (4) when annotation is missing
• WhenNamespaceNotFound - Handles namespace lookup failures
• WhenK8sClientCreationFails - Handles K8s client initialization errors

TestSnapshotLockManager - Tests for per-volume lock management:

• AcquireAndRelease_SingleVolume - Basic lock lifecycle
• AcquireMultipleTimes_SameVolume - Multiple goroutines serialized properly
• AcquireRelease_MultipleVolumes - Independent locks for different volumes
• ConcurrentAccess_SameVolume - Concurrent requests properly serialized
• ReleaseNonExistentLock - Graceful handling of invalid releases

Special notes for your reviewer:

Release note:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deepakkinni

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[k8s-ci-robot]

PR needs rebase.

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