Add v2/completions ingress

What does this MR do and why?

This MR adds /v2/completions route rules to the Nginx ingress. The existing /v1/completions route still works as usual.

graph LR
  classDef subgraph_padding fill:none,stroke:none

  subgraph private k8s
    subgraph 1 [ ]
      ingress(Nginx Ingress)
      subgraph cpu-node-pool
        model-gateway(Model Gateway)
      end
    end
  end

  req((request)) -- "\v2\completions" --> ingress
  ingress --"\v2\completions" --> model-gateway

  class 1 subgraph_padding