Enable removal of `OnionPayload::Invoice::_legacy_hop_data` later

Enable removal of `OnionPayload::Invoice::_legacy_hop_data` later

In fc77c57c3c6e165d26cb5c1f5d1afee0ecd02589 we stopped using the
`FinalOnionHopData` in `OnionPayload::Invoice` directly and renamed
it `_legacy_hop_data` with the intent of removing it in a few
versions. However, we continue to check that it was included in the
serialized data, meaning we would not be able to remove it without
breaking ability to serialize full `ChannelManager`s.

This fixes that by making the `_legacy_hop_data` an `Option` which
we will happily handle just fine if its `None`.