Make `FinalOnionHopData` public

Make `FinalOnionHopData` public

In 4b5db8c3ce, `channelmanager::PendingHTLCRouting` was made
public, exposing a `FinalOnionHopData` field to the world. However,
`FinalOnionHopData` was left crate-private, making the enum
impossible to construct.

There isn't a strong reason for this (even though the
`FinalOnionHopData` API is somewhat confusing, being separated from
the rest of the onion structs), so we expose it here.