Block v3 endpoint (#4629)

## Issue Addressed

#4582

## Proposed Changes

Add a new v3 block fetching flow that can decide to return a Full OR Blinded payload

## Additional Info



Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/http_api/src/validator.rs

@@ -1,5 +1,5 @@
 use beacon_chain::{BeaconChain, BeaconChainError, BeaconChainTypes};
-use types::*;
+use types::{BeaconState, PublicKeyBytes};
 
 /// Uses the `chain.validator_pubkey_cache` to resolve a pubkey to a validator
 /// index and then ensures that the validator exists in the given `state`.