interop/orcalb: Delegate SubConn management to pickfirst

[arjan-bal]

As part of the dual-stack changes, pick_first became the universal leaf policy. All load balancing policies were updated to delegate subchannel management to pick_first (see gRFC A61). The following PRs updated the production LB policies:

• LeastRequest: leastrequest: Delegate subchannel creation to pickfirst #7969
• RoundRobin: roundrobin: Delegate subchannel creation to pickfirst #7966
• RingHash: ringhash: Delegate subchannel creation to pickfirst #8047
• WeightedRoundRobin: balancer/weightedroundrobin: Switch Weighted Round Robin to use pick first instead of SubConns #7826

However, the orcalb used for interop tests still manages subchannels directly: https://github.com/grpc/grpc-go/blob/master/interop/orcalb.go

We should update it to use the endpointsharding balancer, which would, in turn, manage one pick_first instance per endpoint.

[Atul1710]

Hi @arjan-bal, I would like to pick this.

[arjan-bal]

@Atul1710 Sure, I've assigned the issue to you.