CFP: Per-service Load Balancing Algorithm Selection

## Cilium Feature Proposal

Add `service.cilium.io/scheduler` service annotation to dynamically change used load balancing algorithm.

[CFP in Google Docs](https://docs.google.com/document/d/1pDni4HKGWLdg_BEL1cgJkcOZXUjgvihYsrI-EcLCNog/edit?usp=sharing)

Mirrored in https://github.com/cilium/design-cfps/pull/56