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/builder.rs

@@ -880,6 +880,7 @@ where
             // TODO: allow for persisting and loading the pool from disk.
             observed_block_producers: <_>::default(),
             observed_blob_sidecars: <_>::default(),
+            observed_slashable: <_>::default(),
             observed_voluntary_exits: <_>::default(),
             observed_proposer_slashings: <_>::default(),
             observed_attester_slashings: <_>::default(),