QuickGrid: expose list of columns via public method or making protected

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

We’re building a new component that extends the QuickGrid component. Our component inherits from QuickGrid and is adding functionality that we need. Some functionality we’re looking to implement requires our component have the ability to enumerate through the QuickGrid column list and get the column header values.

This presents an issue as the _columns list in QuickGrid.razor.cs is marked private and we’re not able to access it.

### Describe the solution you'd like

- Could this be changed to protected to allow our inheriting class access without making it public?

- Alternatively could there be a public method exposed to return the list of columns.

### Additional context

QuickGrid.razor.cs line 113