"Custom" as third main mode

[kilrau]

Some users prefer to hold all balance in Lightning and prefer to have all mainchain UTXOs automatically swapped to LN. We could:

• let client pass a flag when creating a swap that tells boltz it can delay the claim of the lockup, just like loop does (still tbd if we can do that in cooperative case as it requires ALL clients of the batch to be online and cooperating in the moment of the claim, see Normal Swaps: Batched Key Path Spends boltz-backend#499)
• time low miner fees
• allow manual swap of all funds to LN

Dependencies:

• BTCPay Server 2.0: Megathread btcpayserver/btcpayserver#5899 (comment)

TBD: refunds

[kilrau]

We lateron discussed, that we'd want to extend the third main mode to "Custom" that allows to swap "anything" to either LN or Liquid or maybe even mainchain (no idea who would but we should offer it).