[kbn-grid-layout] Add useCustomDragHandle prop (#210463)

This PR **might** resolve
https://github.com/elastic/kibana/issues/207011 but we will need time
for the telemetry metrics to settle before we know for sure.

## Summary

This PR removes the conditional rendering of the default drag handle in
`kbn-grid-layout`, which has two benefits:

1. It removes the double render of `GridPanel` that was caused by
relying on the `dragHandleCount` to be updated in order to determine
whether the default drag handle should be rendered
2. The default drag handle no longer "flashes" when Dashboards are
loading and waiting for `dragHandleCount` to update
  
   - **Before:**
   

https://github.com/user-attachments/assets/30a032fc-4df3-42ce-9494-dd7f69637c03
      
   - **After:**
   

https://github.com/user-attachments/assets/db447911-cbe2-40dd-9a07-405d1e35a75d


Instead, the consumer of `kbn-grid-layout` is responsible for setting
the `useCustomDragHandle` prop to `true` when they want to use a drag
handle other than the default one.


When adding the `useCustomDragHandle` prop, I got annoyed that I had to
pass this prop all the way down to `grid_panel` - so I decided to swap
to using React context in this PR, as well. The API for the grid layout
component will most likely continue to grow, so this should make it
easier to manage the props.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

examples/grid_example/public/app.tsx

@@ -337,6 +337,7 @@ export const GridExample = ({
               rowHeight,
               columnCount: DASHBOARD_GRID_COLUMN_COUNT,
             }}
+            useCustomDragHandle={true}
             renderPanelContents={renderPanelContents}
             onLayoutChange={(newLayout) => {
               const { panels, rows } = gridLayoutToDashboardPanelMap(