Bluetooth: Mesh: Shell support for remote provisioning

Adds commands in the mesh shell for remote provisioning client
operation, including scanning, provisioning and reprovisioning.

Co-authored-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>
Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>
Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>