LSPS2: Add onion oracle.

[ZmnSCPxj-jr]

Fixes: #44 Rejected?

This is the other pull request. #46 is the other one.

In this particular pull request, we add an onion oracle, i.e. send a notification containing the payment hash and forwarding onion. Of note is that the forwarding onion is needed to support "stateless" invoices where the payment_secret and/or metadata is used to store important payment details. In particular, LDK out-of-the-box supports only stateless invoices (payment preimages are the HMAC of the payment_secret using the node secret key as MAC key). Like #37 (LSPS4) this requires that HTLCs be held at the LSP which the client responds to. Differences are:

• In this pull request even for a multipart payment we only send one notification; LSPS4 gives one notification per HTLC.
• If the client disconnects before it can respond with what to do with the HTLC then the LSP fails the HTLC; LSPS4 re-notifies the client on reconnection, and only fails if it timed out.
• This is specifically only for HTLC; LSPS4 is open-ended enough that it can be used for PTLCs in the future, depending exactly on how it eventually ends up being implemented.

[ZmnSCPxj-jr]

Closing, as this seems to be the least desirable option.