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

`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.

Start by reviewing AppInstance.RedirectActivationToAsync and the AppActivationArguments API, including how activation arguments are currently received and scoped. Define the API changes needed to create or modify arguments for prompts and partial file redirection; done means those scenarios can be supported without relying on the short-lived original arguments.

Written by the indexing model from the issue text.