cluster_header not set for RequestMirrorPolicy

RouteConfiguration always return empty cluster_header fields. Code for reproduce:

```java
RouteConfiguration.newBuilder()
.setName("local_route")
.addVirtualHosts(
VirtualHost.newBuilder()
.addRequestMirrorPolicies(RequestMirrorPolicy.newBuilder().setClusterHeader("cluster-shadow")))
.build();
```
cluster_header exist in snapshot

but not return in RouteConfiguration

```bash
grpcurl -plaintext -d '
{
"version_info":"1",
"type_url":"type.googleapis.com/envoy.config.route.v3.RouteConfiguration",
"resource_names":[
"local_route"
],
"node":{
"cluster":"cluster1",
"id":"id1"
}
}' localhost:8888 envoy.service.discovery.v3.AggregatedDiscoveryService/StreamAggregatedResources | jq .resources[0].virtualHosts[0].requestMirrorPolicies
[
{}
]

```