Bluetooth: Mesh: Add missing model extensions

DFD Server, DFU Server, and DFU Client models are extended models
that extend other underlaying models. If BT_MESH_MODEL_EXTENSIONS
is not enabled, extension hierarchy is not initialized and this
results in issues in subscriptions and errors in representation
of hierarchy in CDP1. Since extensions are spec defined, force
selection of this option here to prevent users forgetting about it.

Signed-off-by: Omkar Kulkarni <omkar.kulkarni@nordicsemi.no>