Support Assigns parameters in exported plugin methods

Plugin authors coming from Xojo use Assigns parameters (parameterized setters, always the last parameter) so users write doc.Metadata("author") = "Björn" instead of a two-argument setter method. Exported C# plugin methods cannot declare that calling convention today. Add an SDK attribute (for example [ObjoAssigns] on a method) marking its last parameter as the Objo Assigns parameter, so the generated API and Studio's compiler accept assignment call syntax for that member.