Add NginxGateway functional test (#2486) #7801

Job	Run time
Checks and variables	1m 38s
NJS Unit Tests	12s
Unit Tests	1m 25s
Build Binary	40s
Build Plus images / build	1m 24s
Build OSS images (nginx, linux/arm64, linux/amd64) / build	1m 18s
Build OSS images (ngf, linux/arm64, linux/amd64) / build	1m 25s
Helm Tests	0s
Conformance tests (nginx, v1.25.16, true) / Run Tests	6m 29s
Conformance tests (nginx, v1.31.0, false) / Run Tests	8m 17s
Conformance tests (nginx, v1.25.16, false) / Run Tests	6m 34s
Conformance tests (nginx, v1.31.0, true) / Run Tests	6m 8s
Conformance tests (plus, v1.25.16, false) / Run Tests	5m 53s
Conformance tests (plus, v1.25.16, true) / Run Tests	13m 17s
Conformance tests (plus, v1.31.0, false) / Run Tests	5m 23s
Conformance tests (plus, v1.31.0, true) / Run Tests	11m 35s
Functional tests (nginx, v1.25.16) / Run Tests	10m 41s
Functional tests (nginx, v1.31.0) / Run Tests	10m 4s
Functional tests (plus, v1.25.16) / Run Tests	10m 22s
Functional tests (plus, v1.31.0) / Run Tests	10m 47s
Package and Publish Helm Chart	0s
	1h 53m 32s

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