Allow replacing output and inserting new outputs

[Kixunil]

This is needed for fancy things like batching or efficient LN channel opening...

[DanGould]

This is supported in PayjoinProposal. What do you have in mind that is not yet possible?

[Kixunil]

Those handle adding inputs, not outputs. I missed substitute_output_address but it really should be substitute_script_pubkey to allow LN channels. Also it'd be great to have methods to reasonably modify values - e.g. giving a discount to the payer.

[jbesraa]

Added this ef8c7ec as part of this pr #233