Bluetooth: Mesh: Add support for Upload OOB Start

This adds support for the Upload OOB Start message to the DFD server, by
providing callbacks that the application can use to hook any OOB scheme
into the model behavior.

There are also extensive changes to the dfu_slot module, to accomodate
the new needs that appeared with the support for OOB transfer (mainly,
fwid, size and metadata are no longer available when the slot is
allocated, they appear later in the handling).

Signed-off-by: Ludvig Samuelsen Jordet <ludvig.jordet@nordicsemi.no>