Manual compaction endpoint backport (#7104)

Backports:
- https://github.com/sigp/lighthouse/pull/7072

To:
- https://github.com/sigp/lighthouse/issues/7039

#7103 should be merged first


  This PR introduces an endpoint that allows users to manually trigger background compaction.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1711,6 +1711,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         }
     }
 
+    pub fn manually_compact_database(&self) {
+        self.store_migrator.process_manual_compaction();
+    }
+
     pub fn manually_finalize_state(
         &self,
         state_root: Hash256,