Payjoin shows an absolute fee when a variable fee may be consumed

[DanGould]

When an original PSBT is sent, indeed, if it were broadcast as-is it would pay a static fee. However the sender includes a recommended additional fee output parameter that allows the receiver to spend some sats from the sender's change (by replacing it with lesser change) in order to pay for a single receiver input of the same type as the sender's for privacy's sake. Perhaps this fee should be reflected in the absolute fee, or the absolute fee should be labeled "at most," or a feerate should be displayed instead.

See the current UI including 159 SATS absolute fee:

Originally posted by @futurepaul in #608 (review)

CC: @benalleng