Implement NKG-specific field validation for Gateway resource #406
Description
Parent issue -- #363
This issue implements requirements of #363 for Gateway resource
Aha! Link: https://nginx.aha.io/features/NKG-43
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Parent issue -- #363
This issue implements requirements of #363 for Gateway resource
Aha! Link: https://nginx.aha.io/features/NKG-43