Bluetooth: Mesh: Allow custom RPL use mesh settings work

This change allows a custom RPL use pending mechanism and the mesh
settings work to store pending RPL entries. `bt_mesh_rpl_pending_store`
is a public API and should be implemented anyway. The custom RPL
implementation has to trigger settings with the
`BT_MESH_SETTINGS_RPL_PENDING` flag to ask the mesh settings work to
call `bt_mesh_rpl_pending_store`.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>