Implement Gateway API resource validation

[pleshakov]

NGINX Kubernetes Gateway (NKG) must validate Gateway API resources for reliability, security and conformity with the Gateway API specification.

See the merged design doc for the background and requirements -- https://github.com/nginxinc/nginx-kubernetes-gateway/blob/4a6b9cd4152ba18dc9a252537526cf73bc07048d/design/resource-validation.md

A/C

• Prepare a design doc for the validation mechanism.
• Implement the validation mechanism.
• Ensure the conformity with the Gateway API by ensuring the Gateway API conformance tests pass for the implemented validation cases.

TO-DO:

• Run webhook validation #362
• Implement NKG-specific field validation #363
• Document how resources are validated #364
• Refactor Webhook validation code #416
• Webhook Validation is bypassed for existing resources when NKG starts #433
• Remove redundant warnings #467
• Make NKG-specific Gateway validation consistent with HTTPRoute #473
• Install the Gateway API Webhook in the installation docs #403
• Validate webhook-related assumptions #515

Aha! Link: https://nginx.aha.io/features/NKG-18