Ability to set backendRef priority level

*Description*:

I have a GRPC service that handles two types of requests:
- large batch requests which are low-priority and can be retried or queued
- individual high priority requests which should always be pushed to the front of the queue

Is there a way to have EnvoyGateway handle this? I think [RoutingPriority](https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/core/v3/base.proto#envoy-v3-api-enum-config-core-v3-routingpriority) could be helpful alongside the `Backend` CRD but instead of failover I just want prioritization.

I'm currently solving this by running multiple deployments of the same service and doing header matching at the route level to split but ideally I'd only have to run one.