Add custom offers

[thomash-acinq]

Allow creating more offers with customizable amount and description.

[pm47]

The catch here is that the invoice is not persisted and would need to be recreated after startup though IIUC.

[thomash-acinq]

Correct

[vincenzopalazzo]

I just noted that this is done inside OfferTypes :)

[vincenzopalazzo]

LGTM, but I left some comments for improving my understanding.

I am building on top of this PR to allow phoenixd to create offers with descriptions.

Following the comments inside the PR storing inside the db the secret: PrivateKey, amount: MilliSatoshi?, description: String? should be enough to recreate the offers right?

[robbiehanson]

I need this for a feature I'm working on in Phoenix (PR #665)

Here's the use case:

• A merchant scans a Bolt Card (v2) with Phoenix app
• The card outputs:
  • encrypted data (for card holder's wallet to decrypt)
  • a method to contact the card holder's wallet directly over the lightning network
• Next the merchant generates a temporary offer (with an amount)
• And then he sends an onion message to the card holder's wallet (which contains his offer, and the card's encrypted data)

So Peer.createOffer is exactly what I need. And for my use case, it doesn't matter if the offer is not persisted.

[vincenzopalazzo]

Hi @robbiehanson I wrote a PR that should be doing this exactly and also working on persistence in #743

Let me know if I need to do something on it that it is not right with the work hat you are doing