Bluetooth: Mesh: DFD Server model

Adds support for the Device Firmware Distribution Server model from Mesh
Model specification v1.1.

Co-authored-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>
Co-authored-by: Ludvig Samuelsen Jordet <ludvig.jordet@nordicsemi.no>
Co-authored-by: Anders Storrø <anders.storro@nordicsemi.no>
Co-authored-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>
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>