Integrate fetching the validator index from BN.

[Zacholme7]

Description

We need to fetch the Validator index for each Validator in the network from the BN. This requires refactoring the entire execution layer to async just to fetch the index, so we decided it is best to push it downstream. Most likely, this will just be some long running task that will periodically fetch the index for all new validators and store it into the database. This is what is done in the go-ssv client for reference.

[dknopik]

The Lighthouse duties services can do this for us! https://github.com/dknopik/anchor/blob/7ce3eb6f665571fbeb117c18711b6cc0bd92463f/anchor/validator_store/src/lib.rs#L508-L526

Basically, I assumed we already had the index (because we have it non-optional in the model), but we can change this impl to update the database 👍

[Zacholme7]

Cool, yep it was the plan to include the index at the EL but didnt pan out so its currently just being defaulted to 0. Easier to give a default value rather than make it optional for state reconstruction on restart.