Add support for bundled payments in BOLT12

[ecdsa]

Add support for bundled payments in BOLT12

[t-bast]

Bolt 12 doesn't support currently this, and won't support this in the MVP. This may come as a follow-up feature, but it definitely needs more spec work to prove that it would work as intended. I had spent some time trying to specify it in more details (see https://gist.github.com/t-bast/69018875f4f95e660ec2cbbc80f711a6) but I'm not sure how to handle the rather annoying partial payment case (where additional on-chain liquidity is added but the "main" payment still cannot be completed, for example because a concurrent payment "stole" the liquidity).

I'm still not entirely convinced that it is absolutely necessary either. In a world where splicing exists, the counterparty risk only exists for the initial channel creation: after that, additional liquidity can be bought trustlessly using the user's channel balance. It seems that paying fees from future HTLCs or using a swap-in to create the initial channel can be an acceptable solution that has a cleaner spec with less edge cases.