Add lifecycle callbacks and EntityCallback support

[juergenzimmermann]

It would be nice if Spring Data R2DBC has hooks like @PostLoad, @PrePersist, @PreUpdate in JPA so that some computation for transient props could take place.

[mp911de]

We will add Spring Data-typical Lifecycle callbacks using application event listeners and the EntityCallback API in a future version. JPA mostly requires lifecycle callback annotations to be used on entities while Spring Data's approach is using external listeners that are also beans.

[juergenzimmermann]

@mp911de Does it mean there will be Interfaces like ReactiveBeforeConvertCallback like in Spring Data MongoDB? If so this would be even better than JPA-like annotations.

[mp911de]

Exactly.

[IvanKonevJr]

Also may be supporting SQL Function around inserting value: #268