create unified slashing cache (#5033)

* create unified slashing cache

* add observed slashable file

* fix broadcast validation tests

* revert block seen cache changes

* clean up slashable cache test

* check header signatures for RPC blobs

* don't throw error on RPC signature invalie

beacon_node/beacon_chain/src/lib.rs

@@ -40,6 +40,7 @@ mod observed_attesters;
 mod observed_blob_sidecars;
 pub mod observed_block_producers;
 pub mod observed_operations;
+mod observed_slashable;
 pub mod otb_verification_service;
 mod persisted_beacon_chain;
 mod persisted_fork_choice;