Add `try-runtime` to Rococo runtime (#6501)

* Add `try-runtime` to Rococo runtime

* Enable try-runtime in all pallets

* Update runtime/rococo/src/lib.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add dev-dependencies for the try-runtime test

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>