Rebalancing traffic from failed backend to healthy one in httproute

Hi! I checked documentation and didn't find any functonality for rebalancing traffic between healthy backends if one of them is going down. I noticed that HTTPBackendRef api spec reference (https://gateway-api.sigs.k8s.io/reference/spec/) have guiding "When a HTTPBackendRef refers to a Service that has no ready endpoints, implementations SHOULD return a 503 for requests to that backend instead", but it's not always preferable action. Our userstory is simple enough: we use gateway api to split incoming traffic by weight via httproute between applications in two data centers, and for us are crucial to lose incoming requests when one of the backends is failing for some reason. We want to be able to enable rebalancing traffic from fully failed backend to healthy one, so (for exaample) 50/50 traffic split becomes 0/100.
I can make a PR with changes that we currently use if it is suitable.