Save FFI attributes in FFIClassAttributes and FFIMethodAttributes

This is for defect [MAGN-3414 Propagate C# attributes through to FEPs] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3414). 

A property ```Attributes``` is added to ```FFIClassAttributes``` and ```FFIMethodAttributes```. In FFI function end point, it could cast its procedure node's ```MethodAttributes``` to ```FFIMethodAttributes``` and then get the corresponding c# attributes.