[FUTURE]: Use default associated type for AuxHandler

 Use NilAuxCommand as default if/when associated type defaults land https://github.com/rust-lang/rust/issues/29661

[Link](https://github.com/NYBACHOK/gears/blob/77c8e4a50af69175c7692188b0d43c23a9775bc6/gears/src/application/handlers/mod.rs#L7-L18)