blip-0036: on-the-fly channel funding

[t-bast]

Payments sent to mobile wallets often fail because the recipient doesn't have enough inbound liquidity to receive it. we add a mechanism to create an on-chain transaction on-the-fly before relaying such payments, which allow them to be relayed once the on-chain transaction is accepted by both peers.

This protocol uses dual-funding, splicing and liquidity ads, leveraging liquidity ads' extensions for paying funding fees.

It is recommended to use 0-conf, to avoid keeping upstream HTLCs held for a long time and locking up liquidity in the network.