Improve custom function in rust

[rifqish]

Function only takes two argument with type ImmutableString.

If i try to invoke a custom function with different type of argument, it said that it can take some other type like 'map'.

Is there any reason you guys limit the usage of the custom function?

[hsluoyz]

The custom functions should support args other than 2. Please refer to other Casbin libs (like Go Casbin, jCasbin, Node-Casbin, etc.) for how they have implemented it

Editor: https://editor.casbin.org/

See docs: https://casbin.org/docs/function