Add gossip validation spec tests for proposer/attester slashings (#9323)

Addresses #9232 partially. This PR covers two topics only.
* #9232

Wires up networking test vectors for `gossip_proposer_slashing` and `gossip_attester_slashing` topics.

The tests also revealed minor spec non-compliance where invalid slashings were ignored rather than rejected.


  - Refactor `process_gossip_proposer_slashing` and `process_gossip_attester_slashing` to return `MessageAcceptance`, so it can be verified in the tests
- Add `GossipValidation` test case, handler, and test entries
- Spec compliance fix: distinguish between internal errors and validation error - return `Reject` when the slashing is invalid and only penalise on invalid messages


Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

beacon_node/network/src/lib.rs

@@ -11,6 +11,7 @@ mod subnet_service;
 mod sync;
 
 pub use lighthouse_network::NetworkConfig;
+pub use network_beacon_processor::NetworkBeaconProcessor;
 pub use service::{
     NetworkMessage, NetworkReceivers, NetworkSenders, NetworkService, ValidatorSubscriptionMessage,
 };