Should optional params Require both `max_additional_fee_contribution` and `additional_fee_output_index`

[benalleng]

Currently the optional parameters for max_additional_fee_contribution and additional_fee_output_index only provide a warning when only one param is present.

rust-payjoin/payjoin/src/receive/optional_parameters.rs

Lines 100 to 107 in eb26494

	match (max_additional_fee_contribution, additional_fee_output_index) {
	(Some(amount), Some(index)) =>
	params.additional_fee_contribution = Some((amount, index)),
	(Some(_), None) | (None, Some(_)) => {
	warn!("only one additional-fee parameter specified: {params:?}");
	}
	_ => (),
	}

In the spec there is a note explaining that they both must be present for the receiver to be able to alter a senders output but it is unclear whether the whole process should halt based on a missing param here.
https://github.com/bitcoin/bips/blob/master/bip-0078.mediawiki#optional-parameters

[DanGould]

postel's law applies here imo since this doesn't create a new spec that might accidentally be conformed to (for example accepting a 1 where 'true' is expected) but it does allow some way to proceed.

A way to document this decision and close the issue is to make a helper fn that explains why we proceed and don't error in docstring.

[zealsham]

Trying to understand this @DanGould and @benalleng

my proposed solution is to have a handle_additional_param method in the impl param block with a verbose docstring describing why we proceed and don't error

[benalleng]

Closed as completed by #853