Add configuration knobs to customize app proxy route selection

[achimnol]

Let's add a configuration knob in the wsproxy.toml to customize the load balancing algorithm for App Proxy.

I think we may introduce per-model-service load balancing customization in the future, so let's name the global configuration as "default routing algorithm" to signify that it's a global fallback.

The prior implementation will be named to "weighted-uniform-random".
The new implementation (#3198) will be named to "shuffled-weighted-round-robin".

This issue also targets to add explicit tests to the routing algorithms.