egctl x translate should be able to process multiple gatewayclasses

*Description*:

For now, the `kubernetesYAMLToResources` function in `egctl x translate` only able to process one GatewayClass, for multiple GC case, the translation will fail.

https://github.com/envoyproxy/gateway/blob/c0068a056f225e136a740823d6d0955cf380374e/internal/cmd/egctl/translate.go#L628

IMO, above function should return something like:

```go
type LoadResources struct {
Resources

GatewayClasses []*gwv1.GatewayClass
EnvoyProxies []*egv1a1.EnvoyProxy
}
```

- The GC and EP should be stored in list instead of variable
- Above function should be placed in gatewayapi layer (This will be super helpful for file-provider in standalone mode to reuse this logic to load all kinds of resources from file)

Any thoughts on this? cc @envoyproxy/gateway-maintainers

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.