Feature request: Allow creation of `AppActivationArguments` for activation redirection

[florelis]

AppInstance.RedirectActivationToAsync() takes an AppActivationArguments. Since that type doesn't have a public constructor, one can only use the arguments that the current instance received, without any modification. As I said in #6074, the activation arguments object has a short lifetime, so redirection can only be done in the short window where the arguments are available. This makes it harder to do initial processing or user interaction before redirection. For example, if for some reason I wanted to show a prompt like "another instance already exists, do you want to open the file in the same window?," or if for file activation I wanted to take the FileActivatedEventArgs.Files and split only a subset of the files to a different instance.

Being able to create AppActivationArguments objects, or to modify the existing one, would make those scenarios possible.