[Feature Request] Health aware weighted clusters: do not route to unhealthy clusters

*Title*: *Health aware weighted cluster routing*

*Description*:
Weighted clusters are frequently used to route to remote ingress endpoints which tend to be single VIPs.
If one of the clusters is "unhealthy" traffic is still sent to the cluster even though it has NO healthy endpoints.

For the above use case this is not the expected or desired behaviour.
So potentially this is a *bug*.

**API change**.
Add a `health_aware_lb` bool to weighted cluster: Do not route to a cluster with no healthy endpoints.

*Implementation Plan*
https://github.com/envoyproxy/envoy/blob/2d3d05dff994eae57b628a765d9cc4958bd1b77e/source/common/router/weighted_cluster_specifier.h#L25

Thread thru `health_aware_lb_` and update `WeightedClustersConfigEntry::clusterWeight` to also take `Upstream::ClusterManager& cluster_manager` and then something like this.

```c++
uint64_t clusterWeight(Runtime::Loader& loader, Upstream::ClusterManager& cm) const {
auto weight = loader.snapshot().getInteger(runtime_key_, cluster_weight_);

if (health_aware_lb_ && weight > 0) {
auto* cluster = cm.getThreadLocalCluster(cluster_name_);
if (cluster != nullptr) {
for (const auto& ps : cluster->prioritySet().hostSetsPerPriority()) {
if (!ps->healthyHosts().empty()) {
return weight;
}
}
return 0;
}
}
return weight;
}
```