Support callback when receiving a PDO

[rohel01]

Initial attempt exposing the slot of the PDO being received

Ideally, I would like to provide &'a [MappingEntry] for the PDO but I am struggling with the current types

pub struct Pdo<'a> {
    // ... Skipped ...

    /// The mapping parameters
    ///
    /// These specify which objects are
    mapping_params: [AtomicCell<Option<MappingEntry<'a>>>; N_MAPPING_PARAMS],
}