Add support for deriving a transient signing pubkey for each Offer from
an ExpandedKey and a nonce. This facilitates recipient privacy by not
tying any Offer to any other nor to the recipient's node id.
Additionally, support stateless Offer verification by setting its
metadata using an HMAC over the nonce and the remaining TLV records,
which will be later verified when receiving an InvoiceRequest.
To support transient signing pubkeys and payer ids for Offers, add
another key derivation to ExpandedKey. Also useful for constructing
metadata for stateless message authentication.
Adds two new payment `Method`s for identifying payments with custom
`min_final_cltv_expiry_delta` as payments with LDK or user payment
hashes.
The `min_final_cltv_expiry_delta` value is packed into the first 2
bytes of the expiry timestamp in the payment secret metadata.
As part of preparing to expose some of its methods as pub for ChannelManager-less
phantom invoice generation.
Pure code move of the module + the addition of module-level documentation
