Hot-reloadable configs inside of Rust: Router

[michaelfeil]

It would be helpful to start some kind of concept of beeing able to hot reload a global config.

This includes in particular: Reloading Router's default temperature or ratios. Under shifting traffic patterns, we might need to tune some knobs. Currently, everything is pretty static, bring up a new router, make sure that the traffic shifts to this.

Would be great to load it from a /configs/router_config.yaml dynamically.

[rmccorm4]

CC @PeaBrane @nnshah1 @grahamking