Add a trait for handling BOLT 12 Offers messages to OnionMessenger and a
skeleton implementation of it for ChannelManager. This allows users to
either provide their own custom handling Offers messages or rely on a
version provided by LDK using stateless verification.
BOLT 12 Offers makes use of onion messages to request and respond with
invoices. Add these types and an error type to OnionMessageContents
along with the necessary parsing and encoding.
