Allow recipeint to contribute fee if receiver sets their own minfeerate

[DanGould]

Now even if a minfeerate is specified by the client the receiver never contributes fees

This is especially critical if the sender's maxadditionalfeecontribution is less than the amount to maintain feerate, either if they don't have enough sats (unlikely) or the receiver wants to add more inputs and outputs to batch transactions

[jbesraa]

Is this still needed? happy to pick it up if so.

[spacebear21]

Closed by https://github.com/payjoin/rust-payjoin/pull/332