Add set_rpc on Field

The __rpc__ attribute is currently filled by method in ModelView. Allowing
fields to define themselves what endpoints should be remotely callable make
this process more flexible.

issue4044
review30991002

trytond/model/fields/function.py

@@ -58,6 +58,9 @@ class Function(Field):
                 return
         setattr(self._field, name, value)
 
+    def set_rpc(self, model):
+        self._field.set_rpc(model)
+
     def sql_type(self):
         return None