Currently the `onion_message` module exposes the blinded route
object as *both* `BlindedRoute` and `BlindedPath`. This is somewhat
confusing, and given they are really paths, not routes (at least in
the sense that a route could be multi-path, though for OMs they are
not), here we unify to only call them paths.
Add a builder for creating offers given a required description and
node_id. Other settings are optional and duplicative settings will
override previous settings for non-Vec fields.
Define an interface for BOLT 12 `offer` messages. The underlying format
consists of the original bytes and the parsed contents.
The bytes are later needed when constructing an `invoice_request`
message. This is because it must mirror all the `offer` TLV records,
including unknown ones, which aren't represented in the contents.
The contents will be used in `invoice_request` messages to avoid
duplication. Some fields while required in a typical user-pays-merchant
flow may not be necessary in the merchant-pays-user flow (i.e., refund).
If we're sending straight to a blinded route with no unblinded intermediate
hops, and we are the introduction node, we need to advance the blinded route by
one hop so that the second hop is the new introduction node.
I'm not sure why rustc didn't complain about the unused parameter
or why we're allowed to get away without explicitly bounding the
`Sign` in the `KeysInterface`, but the current code requires all
`BlindedPath` construction to explicitly turbofish an unused type.
This adds several utilities in service of then adding
OnionMessenger::send_onion_message, which can send to either an unblinded
pubkey or a blinded route. Sending custom TLVs and sending an onion message
containing a reply path are not yet supported.
We also need to split the construct_keys_callback macro into two macros to
avoid an unused assignment warning.
Blinded routes can be provided as destinations for onion messages, when the
recipient prefers to remain anonymous.
We also add supporting utilities for constructing blinded path keys, and
control TLVs structs representing blinded payloads prior to being
encoded/encrypted. These utilities and struct will be re-used in upcoming
commits for sending and receiving/forwarding onion messages.
Finally, add utilities for reading the padding from an onion message's
encrypted TLVs without an intermediate Vec.
