Add NFR scale test to GitHub actions pipeline

[pleshakov]

#1926 adds scale test

To run it successfully by NGF pipeline, the GKE cluster must meet the following requirements:

• A Kubernetes cluster with 12 nodes on GKE
• Node: n2d-standard-16 (16 vCPU, 64GB memory)

Acceptance criteria:

• Ensure scale test successfully runs by NGF pipeline
• Ensure test runs for both NGINX and NGINX Plus images