Nested Mutation Hooks

preCreation / postCreation and preUpdate / postUpdate hooks, designed to allow resolving OpenCRUD nested mutation api, one level at a time (similarly to the way standard apollo server resolvers work on queried nested-entities).

This allows implementing simpler resolving logic, as each mutation resolving has to only deal with the current entity (/level). Useful for implementing authorization checks, auditing entity changes, and more.