Consider Renaming EnvoyProxy API to EnvoyInfra

#871 uses the EnvoyGateway resource to configure the rate limiting infra. The challenge with using EnvoyGateway is that EG must be restarted when changing any configuration. Due to naming, the EnvoyGateway resource was chosen over EnvoyProxy. The name EnvoyProxy implies configuration specific to Envoy proxy. Renaming EnvoyProxy to ProxyInfra will better suit using this resource for managing the rate limiting infra and any other auxiliary control plane. Note that the [infra IR](https://github.com/envoyproxy/gateway/blob/v0.2.0/internal/ir/infra.go#L24-L27) is already designed to support managing multiple types of infra and not just the Envoy proxy infra.