[FluentSplashScreen] Add WaitingMilliseconds and UpdateLabels

[dvoituron]

[FluentSplashScreen] Add WaitingMilliseconds and UpdateLabels

Adding two new properties to allow SplashScreen customization during the loading process:

• SplashScreenContent.DisplayTime: Gets or sets the delay to wait before to close the dialog. Default is 4000 milliseconds.
• SplashScreenContent.UpdateLabels(loadingText, message): Updates the labels of the splash screen.

Example

DialogParameters<SplashScreenContent> parameters = new()
{
    Content = new()
    {
        DisplayTime = 0,    // // <============== See Task.Delay below
        Title = "Core components",
        SubTitle = "Microsoft Fluent UI Blazor library",
        LoadingText = "Loading...",
        Message = (MarkupString)"some <i>extra</i> text <strong>here</strong>",
    }
};
_dialog = await DialogService.ShowSplashScreenAsync(parameters);

var splashScreen = (SplashScreenContent)_dialog.Instance.Content;

// Simulate a first task
await Task.Delay(2000);

// Update the splash screen content and simulate a second task
splashScreen.UpdateLabels(loadingText: "Second task...");   // <==============
await Task.Delay(2000);

await _dialog.CloseAsync();

To solve #1566