Implement Validation Webhook for Splunk Enterprise CRDs

[patrykw-splunk]

Description

Thats a PR to Feature Branch

This PR implements a centralized validation webhook for all Splunk Enterprise CRDs. The webhook validates spec fields on CREATE and UPDATE operations, providing immediate feedback to users when invalid configurations are submitted.

Key Changes

New Validation Package (pkg/splunk/enterprise/validation/):
validator.go - Generic type-safe validator interface using Go generics
registry.go - Validator registry for all CRD types
validate.go - Core validation logic and AdmissionReview handling
server.go - HTTP webhook server with TLS support
common_validation.go - Shared validation functions for common spec fields
CRD-specific validators - Standalone, IndexerCluster, SearchHeadCluster, ClusterManager, LicenseManager, MonitoringConsole

Webhook Infrastructure:
config/webhook/ - Webhook service and ValidatingWebhookConfiguration manifests
config/certmanager/ - TLS certificate configuration via cert-manager
config/default/ - Kustomize patches for webhook integration
cmd/main.go - Webhook server startup integration

Testing and Verification

Unit tests for every change

Related Issues

N/A

PR Checklist

• Code changes adhere to the project's coding standards.
• Relevant unit and integration tests are included.
• Documentation has been updated accordingly.
• All tests pass locally.
• The PR description follows the project's guidelines.

[github-actions]

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❌ @patryk Wasielewski
❌ @patrykw-splunk
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