Add storageSizeMib to DatabaseCluster entity (#330)

* Add `storageSizeMib` to `DatabaseCluster` entity

* Guard against empty list of clusters

* Update DatabaseCluster.php

---------

Co-authored-by: Graham Campbell <[email protected]>

Author: stayallive

src/Entity/DatabaseCluster.php

@@ -60,6 +60,8 @@ final class DatabaseCluster extends AbstractEntity
 
     public string $privateNetworkUuid;
 
+    public ?int $storageSizeMib;
+
     public function build(array $parameters): void
     {
         parent::build($parameters);