reactive lifecycle callback

[gavinking]

It looks like would be relatively straightforward to add support for having the @PrePersist callback and friends return a Uni. To be clear, we would not be able to reuse the code from core, but there's anyway not much of it. We would just implement something from scratch.

The basic idea is to let you write:

@PrePersist
Uni<Void> myCallback() {
     return ..... ;
}

on your entity class.