CPFP: Allow telling coin selector that a candidate has unconfirmed parents

I guess this is just a new field on `Candidate` which lets you say the combined weight and fee of all parents. I think the approach to use in practice in bdk is to roll all inputs that are in an unconfirmed tree into a single candidate. This will give you the correct answer but has the disadvantage of forcing you to spend them altogether. Needs some design work before trying to implement it. Looking at the actual CPFP related logic in core might provide some inspiration.