Add support for custom operations in migration generator #211
Description
We should allow users to execute arbitrary code in the migrations if they need more flexibility than what the existing operations provide. This might be useful not only for some advanced schema manipulation, but primarily for data migrations. A "custom operation" should be provided that just contains a function/closure that executes some code provided by user.
The Cot CLI should be extended with a "make custom migration" command that generates a scaffolding for said feature.