CustomizeLayout method in CliHelpBuilder is not working as expected.

[moh-hassan]

I tried to append a new section to the current generated help using the method CustomizeLayout, but i can't and has no effect.

Something like:

  public void ShowCustomHelp()
  {
      var settings = new CliSettings
      {
          Output = new StringWriter(),
          Theme = CliTheme.Default
      };

      var parseResult = Cli.Parse<RootCliCommand>(["--help"], settings);
      if (parseResult.Action is HelpAction helpAction
          && helpAction.Builder is CliHelpBuilder cliBuilder)
          {
              cliBuilder.CustomizeLayout(context =>
              {
                  var defaultLayout = cliBuilder.GetLayout(context);
                  return defaultLayout.Concat([
                      ctx =>
                      {
                          ctx.Output.WriteLine("This is the footer text.");
                          return true;
                      }
                  ]);
              });

          (helpAction as SynchronousCommandLineAction)?.Invoke(parseResult);
      }

      Console.WriteLine(settings.Output.ToString());
      settings.Output.ToString().Should().Contain("This is the footer text.");
  }

HelpBuilder class in System.CommandLine provide a mechanism to Customize help layout by passing a delegate of type
Func<HelpContext, IEnumerable<Func<HelpContext, bool>>> to the method CustomizeLayout

The customization allowing add or replace a new section to the generated help at runtime.

The Root cause:
The method CustomizeLayout store the getLayout delegate in a private field _getLayout

This private field _getLayout is in the base class HelpBuilder in System.Commandline is not accessible by the class CliHelpBuilder.

CliHelpBuilder need to be modified to support CustomizeLayout

I can provide PR to fix this issue if you like.