Async Payments

[valentinewallace]

Async-receive:

• [ ] Support being the async receiver’s channel counterparty (eg LSP) per the spec PR
  • [ ] Support store-and-forward mailbox for onion messages #2950
  • [ ] ~Track what users we’re waiting to come online (+ persist across restarts)~
  • [ ] Notify sender via OM when receiver comes online
• [ ] Support being the always-online (for now) sender
  • [ ] Notify the receiver’s LSP that an HTLC is being held for the receiver
  • [ ] Send HTLC when notified by the receiver’s LSP
  • [ ] Support manual handling for static invoices
  • [ ] Address https://github.com/lightningdevkit/rust-lightning/pull/3140#discussion_r1760132719
  • [ ] Time out async payments prior to invoice expiry (cc https://github.com/lightningdevkit/rust-lightning/pull/3140#discussion_r1759425911)
• [ ] Support being the async receiver
  • [ ] Support receiving keysend payments to blinded paths #2935, #2956
  • [ ] Support creating a AP BOLT12 offer + keysend invoice that signals that the recipient is often-offline + their LSP’s contact info
  • [ ] Spec the above https://github.com/lightning/bolts/pull/1149
• [ ] Fuzz StaticInvoice message

Async-send:

• [ ] Trampoline #2299
• [ ] Parse AP BOLT12 invoices
• [ ] Support sending an async HTLC and corresponding OM
• [ ] Support retrying the OM or spec out+implement an ACK OM
• [ ] Generate event when an async payment is pending (see https://github.com/lightningdevkit/rust-lightning/pull/3140#discussion_r1673235584)
• [ ] Add RecentPaymentDetails variant for async payments (see https://github.com/lightningdevkit/rust-lightning/pull/3140#discussion_r1651187509)

[TheBlueMatt]

Also https://github.com/lightningdevkit/rust-lightning/pull/3140#discussion_r1760132719 and https://github.com/lightningdevkit/rust-lightning/pull/3140#discussion_r1759425911