cherry-pick rtdb typing fixes from #7422

Author: dconeybe

packages/firebase/compat/index.d.ts

@@ -5819,7 +5819,7 @@ declare namespace firebase.database {
      *   returning true.
      */
     forEach(
-      action: (a: firebase.database.IteratorDataSnapshot) => boolean | void
+      action: (a: firebase.database.IteratedDataSnapshot) => boolean | void
     ): boolean;
     /**
      * Gets the priority value of the data in this `DataSnapshot`.
@@ -5998,6 +5998,10 @@ declare namespace firebase.database {
     toJSON(): Object | null;
   }
 
+  interface IteratedDataSnapshot extends DataSnapshot {
+    key: string; // key of the location of this snapshot.
+  }
+
   /**
    * The Firebase Database service interface.
    *