Persist data columns to store (#6255)

* Persist data columns (from das PR #5196)

beacon_node/beacon_chain/src/data_column_verification.rs

@@ -250,6 +250,10 @@ impl<E: EthSpec> KzgVerifiedCustodyDataColumn<E> {
     pub fn into_inner(self) -> Arc<DataColumnSidecar<E>> {
         self.data
     }
+
+    pub fn as_data_column(&self) -> &DataColumnSidecar<E> {
+        &self.data
+    }
 }
 
 /// Complete kzg verification for a `DataColumnSidecar`.