fix: Update IrisGridContextMenuHandler getHeaderActions return type to be more permissive (#2117)

Needed for proper TS in
deephaven/deephaven-plugins#522. Otherwise the
class complains it can't properly extend `IrisGridContextMenuHandler`.

Author: mattrunyon

packages/iris-grid/src/mousehandlers/IrisGridContextMenuHandler.tsx

@@ -177,7 +177,7 @@ class IrisGridContextMenuHandler extends GridMouseHandler {
   getHeaderActions(
     modelIndex: ModelIndex,
     gridPoint: GridPoint
-  ): ContextAction[] {
+  ): ResolvableContextAction[] {
     const { irisGrid } = this;
     const { column: visibleIndex } = gridPoint;
     assertNotNull(visibleIndex);