feature request: ring_hash as a general load balancer

The ring_hash load balancer was designed in [gRFC-A42](https://github.com/grpc/proposal/blob/master/A42-xds-ring-hash-lb-policy.md) for xDS session affinity. Some users want to use ring_hash as general load balancer without being part of xDS. This might need a cross-language design (gRFC) on the general purpose ring_hash lb policy first.