Add notifications to the delivery api for further extension

[Yannis-S-Standaert]

In some cases, it would be great to be able to hook into certain processes in the delivery api
one such example could be:

ContentDeliveringNotification -> returns a content model with a modifyable list of properties, so we can add or remove properties just before the api returns them.

Right now, there are many cases where i would like to enrich the data of piece of content with dynamic data, but right now it does not seem to be possible?