Adds a new `/lighthouse` API call to the VC which allows the list of beacon nodes to be updated dynamically at runtime.
An entirely new beacon node list is provided to the VC so it effectively adds, removes or reorders nodes to match the new list.
This can then be used in Siren, which will enable a "drag to reorder" system along with adding and removing beacon nodes while the VC is on. This will make it unnecessary to reboot the VC when users want to simply add or remove a BN from the list.
- Create trait `ValidatorStore` with all functions used by the `validator_services`
- Make `validator_services` generic on `S: ValidatorStore`
- Introduce `LighthouseValidatorStore`, which has identical functionality to the old `ValidatorStore`
- Remove dependencies (especially `environment`) from `validator_services` and `beacon_node_fallback` in order to be able to cleanly use them in Anchor
