Configurable chat toolbar in side panel

[brichet]

This PR adds a chatToolbarItems to the MultiChatPanel, to automatically add button to the toolbar when opening.

Adds a toolbar factory to the multichat panel, that is passed to the chat widget opened in the panel.
In jupyterlab-chat, the toolbar factory is built from the settings schema, and therefore have the same items as in the main area chat.

Some more context at #266 (comment)

API CHANGE

Since the same toolbar factory can be used in a main area chat and a chat in the side panel, this PR introduce a new interface to unify a chat panel (basically a panel containing a chat widget and a toolbar):

/**
 * The interface for any widget displaying a chat (main area or side panel).
 */
export interface IChatPanel extends Widget {
  /**
   * The chat widget embedded in the panel.
   */
  widget: ChatWidget;
  /**
   * The model of the chat widget.
   */
  model: IChatModel;
  /**
   * The area of the panel.
   */
  area: ChatArea;
  /**
   * The chat panel toolbar.
   */
  toolbar: Widget;
}

This widget interface is used to build the toolbar items in the registry, whether the widget is a main area chat or a side panel chat.

/**
 * An optional toolbar factory for each opened chat.
 */
 chatToolbarFactory?: (
   panel: IChatPanel
 ) => IObservableList<ToolbarRegistry.IToolbarItem>;

This interface is also used as the tracked widget from the chat tracker (API change):

/**
 * the chat tracker type.
 */
- export type IChatTracker = IWidgetTracker<ChatWidget | MainAreaChat>;
+ export type IChatTracker = IWidgetTracker<IChatPanel>;

Any extension bumping to the next version of @jupyter-chat and providing an IChatTracker should update its widgets accordingly.

[github-actions]

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