Reorg events (#2090)

## Issue Addressed

Resolves #2088

## Proposed Changes

Add the `chain_reorg` SSE event topic

## Additional Info


Co-authored-by: realbigsean <seananderson33@gmail.com>
Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/beacon_chain/src/errors.rs

@@ -117,6 +117,10 @@ pub enum BeaconChainError {
         request_slot: Slot,
         slot: Slot,
     },
+    InvalidReorgSlotIter {
+        old_slot: Slot,
+        new_slot: Slot,
+    },
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);