Bluetooth: Mesh: DFU models

Adds DFU models from the Mesh Model specification v1.1.

Co-authored-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>
Co-authored-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>
Co-authored-by: Anders Storrø <anders.storro@nordicsemi.no>
Co-authored-by: Mia Koen <mia.koen@nordicsemi.no>
Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>
Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>