LSPS1: Use a separate fee for onchain vs ln payments

[SeverinAlexB]

LSPS1 uses one field fee_total_sat to indicate how much the user needs to pay for the requested channel.

Issue: With rising onchain fees, onchain payments are exceedingly more expensive for the lsp than lightning payments. Onchain payments create its own UTXO and are therefore more expensive for the LSP to spend.

Solution: Add a different price for LN payments compared to onchain payments. This can be done directly now or in a backwards compatible way at a later point.

[tnull]

Makes sense to me, probably best to do it now.

[ErikDeSmedt]

Having a separate fee for on-chain is a good idea.

If we change the spec I'd like to consider how we could include dual funding as a payment option.
The proposal is final and has been merged a 2 weeks ago.