External File Storage: Allow to suggest a filename in SaveFile

[IceOnly]

Summary

Allows to predefine the FileName:

trigger OnAction()
var
    ExternalFileStorage: Codeunit "External File Storage";
    Path: Text;
begin
    ExternalFileStorage.Initialize("File Scenario"::Default);
    Path := ExternalFileStorage.SaveFile('2023', 'Invoice_1234', 'pdf', 'Save invoice...');
end;

Work Item(s)

Fixes #3272

Fixes AB#571521