Fix slasher OOM (#9141)

Fix a vulnerability in the slasher whereby it would OOM upon processing an invalid attestation with an artificially high `validator_index`. This fix has already been made available to affected users on the `slasher-fix` branch.


  - Prevent attestations from being passed to the slasher prior to signature verification. This was unnecessary, as they would later be passed on successful validation as well.
- Add a defensive cap on the maximum validator index processable by the slasher. The cap is high enough that it shouldn't be reached for several years, and will quickly result in warning logs if forgotten.
- Add a regression test that confirms that the issue is fixed.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/beacon_chain/src/attestation_verification.rs

@@ -514,11 +514,6 @@ impl<'a, T: BeaconChainTypes> IndexedAggregatedAttestation<'a, T> {
         chain: &BeaconChain<T>,
     ) -> Result<Self, Error> {
         Self::verify_slashable(signed_aggregate, chain)
-            .inspect(|verified_aggregate| {
-                if let Some(slasher) = chain.slasher.as_ref() {
-                    slasher.accept_attestation(verified_aggregate.indexed_attestation.clone());
-                }
-            })
             .map_err(|slash_info| process_slash_info(slash_info, chain))
     }
 
@@ -971,11 +966,6 @@ impl<'a, T: BeaconChainTypes> IndexedUnaggregatedAttestation<'a, T> {
         chain: &BeaconChain<T>,
     ) -> Result<Self, Error> {
         Self::verify_slashable(attestation, subnet_id, chain)
-            .inspect(|verified_unaggregated| {
-                if let Some(slasher) = chain.slasher.as_ref() {
-                    slasher.accept_attestation(verified_unaggregated.indexed_attestation.clone());
-                }
-            })
             .map_err(|slash_info| process_slash_info(slash_info, chain))
     }