fundpsbt should add fee output for elements tx

[niftynei]

fundpsbt should add (or merge, as required) a fee output for elements txs -- it's required for validation and sending.

[ZmnSCPxj]

It would not be able to know the fees of the outputs that will be added later.

See this alternative API I proposed: #3838 (comment)

With the latter API, we can determine the fees for the outputs as well as the fee output.

[cdecker]

It doesn't really have to know the fees that any later addition of inputs would result in, as long as each external tool adds or subtracts their fees to the existing fee output. I like the idea of having the psbt representation be as consistent as possible at all times.

[ZmnSCPxj]

But it does have to know the fees of additional outputs, because it has to allocate inputs that can cover those fees as well.

[niftynei]

supersumed by #3998