Support knob to configure consistent hash table size

[aoledk]

Description:

Backend whose LoadBalancer type is ConsistentHash will be translated to xDS cluster whose type is Maglev, but we can't configure maglev table size which is very important in long lived TCP load balancing use case. Bigger size, smaller disruption.

[optional Relevant Links:]

https://gateway.envoyproxy.io/v1.0.1/api/extension_types/#consistenthash https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/cluster/v3/cluster.proto#config-cluster-v3-cluster-maglevlbconfig

[aoledk]

/assign

[shawnh2]

reopen this since #3348 only implements api