Support hashing on resources field in addition to Node in NodeGroup

I've been thinking about how to improve the performance of our control plane, and I've realized that with EDS in particular, the control plane does a lot of work generating ClusterLoadAssignments that could theoretically be shared for many different Nodes.

If 3 different microservices depend on the "foo" cluster for example, we can likely generate just one ClusterLoadAssignment for "foo" one time and share it for all.

Right now we're using ADS for everything, I'm thinking about using ADS just for Listeners, Routes, and Clusters, and having a separate EDS only control plane that can create a NodeGroup based on the resource names (from DiscoveryRequest) for the ClusterLoadAssignment in addition to the Node.

What do y'all think about me adding that capability?

Stated another way, instead of only providing

```
public interface NodeGroup {
T hash(Node var1);
}
```

We could also allow implementing

```
public interface NodeAndResourceNamesGroup {
T hash(Node var1, Set resourceNames);
}
```