support customizing envbox host files via host_aliases

[ericpaulsen]

a customer wants to set host_aliases in their envbox-based workspace. they mention this works for non-envbox workspaces. K8s docs here. below is what they've set in their template:

  spec {
    restart_policy = "Never"

    host_aliases {
      hostnames = ["…."]
      ip        = "127.0.0.1"
    }