HorizontalPodAutoscaler Support

[mpstefan]

As a cluster operator using NGF
I want NGF to work with a HorizontalPodAutoscaler
So that I can automate the scaling of my data planes to save on resources and scale up when traffic increases.

NIC PR: nginx/kubernetes-ingress#3276

Acceptance

• The user can setup a HorizontalPodAutoscaler for an NGINX deployment (data plane).
• The user can setup a HorizontalPodAutoscaler for the NGF control plane.
• Update the scaling doc to talk about how to use the HorizontalPodAutoscaler.

[mpstefan]

Original discussion issue: #3440

@nowjean Moved your original question to be tracked in this issue. Thanks for the feedback!

[nowjean]

@mpstefan Thank! I'm using NGF2.0. For HPA, Do I have to apply hpa to control plane(ngf-nginx-gateway-fabric) or data plane(practice-gateway-nginx) or both of them?