fix: Update IrisGridContextMenuHandler getHeaderActions return type t…

…o be more permissive (#2117)

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

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);