Add default backendRefs to the BackendTrafficPolicy

*Description*:

Adding an HTTP Route for a particular hostname invalidates other routes, which have a match on the context path and headers but use wildcard hostname(s). This makes the creation of a "catch-all" HTTP Route impossible. Such a route can be used when two ingress systems are present in the cluster, for example, for migration to EG.

When a default `backendRefs` is present in the attached BackendTrafficPolicy, instead of throwing `NR`, EG can forward the request to it

*Relevant Links*:

Please, see the discussion in:
* #5500