linkerd-failover extension should support HTTPRoute for pod-to-pod cluster linking

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### What problem are you trying to solve?

Linkerd 2.14 introduced pod-to-pod cluster linking, which simplifies setup and removes need of additional Load Balancers. With this change configuration of routing of traffic to pods running in a different cluster is done using `HTTPRoute` objects. Use of `TrafficSplit` is no longer supported in this setup, so it's impossible to set up a configuration where all requests are handled locally and routed to a different cluster only when local pods are unavailable (i.e. failover to different cluster).

### How should the problem be solved?

Please update `linkerd-failover` extension to monitor and update the `HTTPRoute` objects in a manner similar to how it worked with `TrafficSplit`:
- monitor `HTTPRoute` objects with proper label
- allow setting one backend with weight `1` and other with weight `0`
- let `linkerd-failover` extension to change weights when local backend becomes unavailable

### Any alternatives you've considered?

It is possible to set backend weights in big disproportion, e.g. 99 and 1, but this would not be a true failover, as 1% of the traffic would always be routed to a different cluster.

### How would users interact with this feature?

This would be in-place replacement of current setup which uses `TrafficSplit`, so no change in how to work with this functionality.

### Would you like to work on this feature?

no

Start by reading the linkerd-failover extension's existing TrafficSplit handling and the Kubernetes HTTPRoute API. Implement equivalent monitoring and weight updates for labeled HTTPRoute objects, including 1/0 backend weights and failover changes, then verify that local traffic remains preferred until the local backend is unavailable.

Written by the indexing model from the issue text.