Proposal: Bundled payments

In an email exchange between Thomas and David, they discuss the semantics of bundled payments. Thomas mentions that for the feature to be optional, the prepayment amount needs to be subtracted from the main payment amount. He provides an example where Alice would expect to receive either 1 BTC with the invoice payment hash or 1 BTC minus 10k sats with the invoice payment hash, along with 10k sats via keysend with the prepayment hash.David responds to Thomas's email and asks if it would be acceptable for the prepayment to use a keysend payment instead of a BOLT11 invoice. He suggests using the onion message payload to indicate which payment hash to associate with the prepayment. David proposes extending the BOLT11 invoices with an extra_fee_via_keysend field, which would be smaller and allow encoding in existing BOLT11 fields like description or metadata.The email exchange ends with Thomas's signature information: Electrum Technologies GmbH / Paul-Lincke-Ufer 8d / 10999 Berlin / Germany, Sitz, Registergericht: Berlin, Amtsgericht Charlottenburg, HRB 164636, Geschäftsführer: Thomas Voegtlin.