Reuse Inner HBONE connection when sending cross cluster traffic

[Stevenjin8]

Right now, we establish a new inner HBONE tunner when sending traffic across cluster. This somewhat defeats the point of HBONE and is expensive as it requires a new TLS/H2 handshake.

Doing this correctly is a little tricky since we do split horizon wds. So a naive approach of just throwing stuff in connection pools will make it so we always hit the same backend. Not good.

I think a good place to start it to establish up to O(N) hbone inner hbone tunnels before pooling where N is the weight of the split horizon wds.

[keithmattix]

Is this basically like Envoy's preconnect for ztunnel?