grpc/child_manager: use generic child builder

[arjan-bal]

Contributes to: #2761

Make ChildManager generic over the LbPolicyBuilder type it manages.

Previously, type checking for LB policies and their configuration types relied on runtime invariants enforced by DynAdapter. Parameterizing ChildManager provides compile-time guarantees for these types.

As a result:

• RoundRobin now manages concrete, non-type-erased PickFirst children.
• Priority will manage GracefulSwitch children.

Additional Changes

• Updated RoundRobin tests to use the real PickFirst balancer rather than test stubs.

[dfawley]

After chatting with @LucioFranco, it's probably best to remove the auto_impl usage for now. It seems like people take the proc macro costs pretty seriously, and we also might be able to tweak the way we use serde to avoid it, too... But if we start adopting this crate, then we'll have more work to do if we do want to do that.

[dfawley]

Everything else LGTM, so approving for when that's removed.