Improve transaction builder APIs around SCIs

[cbeck88]

Maybe it makes sense to change the transaction builder api (in a separate pr)?
My thinking is that since this API is always used by the claimer of an SCI, wouldn't it make sense that they always specify how much they want to get? I think they'll always start with that so they'll end up doing this exact subtraction every time this is used.

Originally posted by @eranrund in #3214 (comment)

There are two things I want to change about the transaction builder APIs around SCIs:

1. Instead of passing the "partial fill change value", instead pass this "partial fill value" which is intuitively what the user is trying to take from the swap, and let the supporting code calculate the change outputs under the hood.
2. Instead of calling it "add_presigned_input" and "add_presigned_partial_fill_input", just call it "add_signed_contingent_input" and "add_partial_fill_signed_contingent_input". I don't know why I introduced a new term for this, it's needlessly confusing and I should have tried to be consistent.