Implement e2e tests for network policy in API Gateway

[Ressetkk]

As a result of #2523, implement E2E test to validate connectivity of the API gateway in NetworkPolicy-configured cluster.

• Deploy deny-all network policy in kyma-system
• check if connectivity is working when networkPoliciesEnabled is set to true in APIGateway CR
• set networkPoliciesEnabled to false in APIGateway CR
• check if connectivity is working (APIServer, webhooks, metrics endpoint, DNS resolution)