core: add subchannel list LB commons utility function, refactor for WRR

[YifeiZhuang]

see details in go/grpc-java-wrr(first part)

[ejona86]

I expect we'll want to morph this utility in one of two opposite ways:

1. Have it be more a library that is called. So instead of it calling a function to create the picker the LB would call a method to get the list of subchannels and pass that to a picker
2. Have it be the full LB, but sort of an adapter to a simplified interface that mainly creates the picker given a list of subchannels.

I think things are still clear enough with this current state that we can move forward and see how things go and see which direction will work out better.

[YifeiZhuang]

2. Have it be the full LB, but sort of an adapter to a simplified interface that mainly creates the picker given a list of subchannels.

For 2 it becomes a ForwardingLoadBalancer delegate to this, isn't it.

[ejona86]

For 2 it becomes a ForwardingLoadBalancer delegate to this, isn't it.

Actually, I'm thinking even further where it is the LB itself and RRLB goes away and is just a RRSimpleLB or something. The RRProvider would construct the utility as the LB. This approach would basically give us a "simple LB API" that is easy and this utility would be an adapter to the "full LB API." Sure, if needed we could wrap it in a ForwardingLoadBalancer, but that might mean "the approach isn't working."