cleanup: Add child balancer in stub.BalancerData

[arjan-bal]

Almost all references of BalancerData.Data are storing child balancers in it. This field is of type any, so callers have to cast it to a balancer.Balancer before every access. Example:

grpc-go/internal/idle/idle_e2e_test.go

Lines 103 to 106 in 4275c5b

	UpdateClientConnState: func(bd *stub.BalancerData, ccs balancer.ClientConnState) error {
	bal := bd.Data.(balancer.Balancer)
	return bal.UpdateClientConnState(ccs)
	},

We should introduce a new field of type Balancer to make using the stub balancer more ergonomic. We should update existing usages use the new field wherever possible.

[vinothkumarr227]

@arjan-bal Please assign the issue to me.