Modularize validator store

[dknopik]

This is a draft PR for modularizing the validator store so that we can reuse the validator services in Anchor. It is very work in progress. :)

Proposed Changes

• 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

Additional Info

This is dependent on the tracing integration changes (#6339) - for development, I pulled in some changes manually in the required places. This breaks Lighthouse, so I have not yet tried running Lighthouse with these changes.