allow deploying other GatewayClasses

[stevenctl]

Description

Allows configuring the our gateway controller to deploy gateways with additional classes.
Reduced-scope version of solo-io#9471

API changes

The only API is an internal Go API to allow GME to take control of the Gateway translation.

// bootstrap opts
ExtraGatewayClasses []string

Code Changes

The translator will now ignore other non gloo-gateway gateways (separate translation should be added for them, this PR just allows these classes to be deployed)

Also, fixed an issue where not fully specifying pull-policy breaks the deployer

{"controller": "gateway", "controllerGroup": "gateway.networking.k8s.io", "controllerKind": "Gateway", "Gateway": {"name":"gw-clsname","namespace":"default"}, "namespace": "default", "name": "gw-clsname", "reconcileID": "29e3d9fe-2821-424c-a4b0-1ee4f2d855db", "error": "failed to apply object apps/v1, Kind=Deployment gloo-proxy-gw-clsname: Deployment.apps \"gloo-proxy-gw-clsname\" is invalid: spec.template.spec.containers[0].imagePullPolicy: Unsupported value: \"Unspecified\": supported values: \"Always\", \"IfNotPresent\", \"Never\""}

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works