FilePicker default directory

### Description

I would love to have the ability to set a default directory in the FilePicker type. i.e. Set FilePicker to a specific directory when it displays to the user.

### Public API Changes

public class PickOptions
{
public PickOptions();

public static PickOptions Default { get; }


public static PickOptions Images { get; }

public string? PickerTitle { get; set; }

public FilePickerFileType? FileTypes { get; set; }

--> public string BaseDirectory? {get; set;}
}
}

### Intended Use-Case

In my application I have the ability for users to Open and Backup/Restore project files. Currently when "Open Project" is selected the FilePicker opens in the Documents directory. Being able to set a default directory for the picker to open too would save a lot of support issues once a production release is pushed out.

Start by locating the FilePicker type and the PickOptions class described in the issue, then trace how picker options reach each supported platform. Define the default-directory behavior for the requested property and verify that existing picker title and file-type behavior remains unchanged across platforms.

Written by the indexing model from the issue text.