Feat: Add TemplateColumn<TModel, TValue> for per-cell data

[fidarit]

This PR adds a new column type TemplateColumn<TModel, TValue> to TreeDataGrid. Unlike the existing TemplateColumn<TModel>, this one lets each cell have its own typed value (TValue) rather than using the whole row model. This makes it easier to work with rows that contain multiple independent objects or properties.

• Each cell has its own data context provided by the getter function.
• You can define per-cell read-only rules using TemplateColumnOptions<TValue>.
• Constructor uses a resource key for templates instead of IDataTemplate to keep UI in XAML and maintain MVVM compliance.

Example

new TemplateColumn<DataRow, DataCell>(
    "Cell 0",
    row => row.Cells[0],
    "CellTemplate",
    "CellEditTemplate",
    options: new TemplateColumnOptions<DataCell>
    {
        IsReadOnlyGetter = cell => cell.IsLocked
    });