Add support for column commands and download source code.

[Jayson Maxson (jmaxson-ms)]

What's new

New namespace: Microsoft.Performance.SDK.ColumnCommands

• DataColumnCommands — sealed container that carries the optional commands available on a column. Exposes:
• DataColumnCommands.Empty
• TryGetDownloadSourceCodeCommand(out DownloadSourceCodeCommand)
• DownloadSourceCodeCommand — abstract base a plugin implements to advertise "download source code" for a column value:
• string CommandName
• bool CanExecute(object value, string downloadPath)
• Task ExecuteAsync(object value, string downloadPath, CancellationToken)
• DownloadSourceCodeResult — result type with a success Uri (typically a local file URI the host opens) or a failure with ErrorMessage and optional attempted Uri.
  New interface: IDataColumnCommands
• Implemented by DataColumn and HierarchicalDataColumn so hosts can retrieve the DataColumnCommands associated with any column.

Column builder integration

• ColumnBuilder.WithCommands(DataColumnCommands) — attach commands to the built column, mirroring the existing WithVariants(...) pattern.
• New WithToggle(..., DataColumnCommands) and WithHierarchicalToggle(..., DataColumnCommands) overloads on ToggleableColumnBuilder, plus equivalent overloads on ModalColumnBuilder and HierarchicalColumnBuilder, so commands can also be scoped to a specific variant.
• DataColumn / HierarchicalDataColumn constructors accept an optional DataColumnCommands, and the runtime builders (EmptyColumnBuilder, ToggledColumnBuilder, ModalColumnWithModesBuilder) flow it through when materializing columns.

Design notes

• DataColumnCommands is non-generic. The commands operate on object value because for hierarchical columns the row value may differ from T due to an ICollectionAccessProvider<T, TOut>. This is documented on the type.
• DownloadSourceCodeCommand itself is non-generic for the same reason and to keep the host-side dispatch simple.
• Commands were placed on the column (via IDataColumn / IHierarchicalDataColumn) rather than on ITableBuilder, because they are intrinsic column data (like ColumnConfiguration or the projection). The tradeoffs are captured inline in DataColumnCommands.cs.