Build properties

Here it is !

The underlying wiring is done: process -> data -> db.

I don't have data endpoint yet, it should make the subject of another PR I believe, That one is already big enough ...
I would separate the properties handling from the build api. It might not be the case for buildrequest, but that does not mean this is the way to go. build properties can change during the build lifecycle, which is not the case for buildrequest. So build properties deserve their own data endpoints

There is already a data/properties.py module which only waits to get more implementation.