Option to have upcoming duties as metrics

[ybstaked]

For block proposals, we should use /eth/v1/validator/duties/proposer/<epoch> where we can look 1 epoch into the future which gives us 6.4-12.8 minutes to respond. We can look at either pubkey or index.

https://ethereum.github.io/beacon-APIs/#/Validator/getProposerDuties

Sync committees are very similar, we should use /eth/v1/validator/duties/sync/<epoch> where we should be able to look up to 256 epochs into the future.

https://ethereum.github.io/beacon-APIs/#/Validator/getSyncCommitteeDuties