e2e/operator: Refactor encryption provider interface and fix single-cluster setup

[nameisbhaskar]

Introduce clean interface-based architecture for encryption at rest testing:

Core Changes:

• Create encryption.Provider interface with platform-specific config
• Define encryption.PlatformConfig (Platform, RequiresCredentialsSecret, etc.)
• Add GetEncryptionProvider() to CloudProvider interface
• Auto-set encryption provider in ProviderFactory

Fix: Cloud-Specific Config Flow

• Problem: Platform/credentials hardcoded to "UNKNOWN_KEY_TYPE"
• Solution: EncryptionAtRestConfig() dynamically retrieves from provider
• Platform and cmekCredentialsSecretName now flow correctly to helm charts
• Enables AWS_KMS, GCP_CLOUD_KMS, UNKNOWN_KEY_TYPE support

Additional Improvements:

• Extract shared SetupFileBasedEncryptionSecrets() utility
• Remove unused IsMultiRegion field and OperatorUseCases interface
• Add utils.GetProviderFromEnv() and utils.GenerateClusterNames()
• Better cluster naming: PR numbers in CI, usernames locally

Fix Kind single-cluster deployments timing out during infrastructure setup:

Problem: CoreDNS LoadBalancer service waited for IP without MetalLB Solution: Skip MetalLB and CoreDNS for single-cluster (uses default DNS)

Single-cluster now completes setup in seconds instead of timing out.

• tests/e2e/operator/encryption/types.go: Provider interface
• tests/e2e/operator/infra/{provider,local,gcp}.go: Implementations
• tests/e2e/operator/region.go: Fix EncryptionAtRestConfig()
• tests/e2e/operator/utils/cluster_naming.go: New utilities
• tests/e2e/operator/*/test.go: Use new architecture