Add a plug that detects the need for running mix ash.codegen

[zachdaniel]

There are some challenges:

1. AshPostgres and AshGraphql both expect to be able to exit/raise. We need to make sure that they raise a specific known error in this case, and don't use exit when we are calling their codegen functions.
2. In order to run mix ash.codegen, we need to provide a name. The current tooling in Plug.Exception supports actions, but not actions that take inputs. Perhaps we need to try and improve Plug.Exception to support inputs. I find it unlikely that it would be something that they are open to but worth checking.