Support DLT Header only messages (+ add `DltPacketSlice::message_id_and_payload`)

• DltPacketSlice::from_slice & storage::DltStorageReader now allow DLT packets that only consist of an header.
• Added new method DltPacketSlice::message_id_and_payload that allows to get the message id together with the non verbose payload.
• DltPacketSlice ::non_verbose_payload return type changed from &[u8] to Option<&[u8]> (only return Some if the packet is non_verbose and the length is big enough.