QuickGrid: Make ColumnBase.IsSortableByDefault() public.

## Background and Motivation

QuickGrid's ColumnBase currently has a HeaderTemplate property that allows you to render a custom header for the column. The default header is rendered in ColumnBase.RenderDefaultHeaderContent(RenderTreeBuilder) and makes use of the method IsSortableByDefault(), which is protected. Because of this, it's not possible to properly check if the column is sortable when using a custom header.

## Proposed API

```diff
namespace Microsoft.AspNetCore.Components.QuickGrid;

public abstract partial class ColumnBase
{
- protected virtual bool IsSortableByDefault();
+ public virtual bool IsSortableByDefault();
}
```

## Usage Examples

## Alternative Designs

## Risks