xds: support ring_hash as the endpoint-level LB policy

[voidzcy]

To be merged after #7943.

There two main parts of this change:

• Changed the CDS LB policy to accept "ring_hash" as the endpoint LB policy from CDS updates. This configuration is directly passed to its child policy (aka, ClusterResolverLoadBalancer) in its config.
  • Note, for underlying clusters of an aggregate cluster, its endpoint-level LB policy is configured by the top-level cluster configuration.
• Changed ClusterResolverLoadBalancer to generate different LB configs for its downstream LB policies. Depending on the endpoint-level LB policies, the downstream LB tree hierarchy will be different.
  • If the endpoint-level LB policy is "round_robin", the downstream LB policy hierarchy is: PriorityLB -> ClusterImplLB -> WeightedTargetLB -> RoundRobinLB (same as existing).
  • If the endpoin-level LB policy is "ring_hash", the downstream LB policy hierarchy is: PriorityLB -> ClusterImplLB -> RingHashLB. That is, endpoints in different localities are mixed together for load balancing. Each endpoints will be attached with a weight value that is the product of its endpoint-level weight and the weight of the locality it belongs to.

[voidzcy]

This can be reviewed and merged now.

[dapengzhang0]

LGTM