feat(gcpnfsbackup): change AccessibleFrom to discriminated union struct

[dushanpantic]

Description

Refactors accessibleFrom field in GcpNfsVolumeBackup and GcpNfsBackupSchedule from []string with magic "all" value to a discriminated union struct with type and targets fields.

Motivation
The previous API used a magic string "all" mixed with regular shoot IDs, which had several issues:

• Poor discoverability: Users had to know the magic value exists
• Complex validation: CEL rules needed to check "all" isn't combined with other values
• No IDE support: No autocompletion or type hints for valid options
• Ambiguous semantics: Is "all" a shoot name or a special keyword?

The new design follows established Kubernetes API patterns where a type field discriminates behavior:

K8s Resource	Type Field	Pattern
Service	type: ClusterIP|NodePort|LoadBalancer	Type determines which fields are valid
Ingress	pathType: Exact|Prefix|ImplementationSpecific	Type changes path interpretation
HPA	metrics[].type: Resource|Pods|Object|External	Type selects metric source struct

Changes proposed in this pull request:

• API: New AccessibleFrom struct with Type enum (All/Specific) and Targets slice
• CEL validation ensures targets required when Specific, empty when All
• Updated SKR reconcilers (v1/v2), backup schedule, test DSL, and controller tests
• Updated user documentation

Migration
Before:

accessibleFrom: ["all"]

After:

accessibleFrom:
  type: All

Before:

accessibleFrom: ["shoot-1", "shoot-2"]

After:

accessibleFrom:
  type: Specific
  targets:
    - shoot-1
    - shoot-2

Related issue(s)

[hyperspace-insights]

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