feat: make execution_graph.stages() public (#1256)

* make stages() from execute graph public
* add rust doc to exposed public method

Author: milenkovicm

ballista/scheduler/src/state/execution_graph.rs

@@ -218,7 +218,8 @@ impl ExecutionGraph {
         new_tid
     }
 
-    pub(crate) fn stages(&self) -> &HashMap<usize, ExecutionStage> {
+    /// Exposes executions stages and stage id's
+    pub fn stages(&self) -> &HashMap<usize, ExecutionStage> {
         &self.stages
     }