[specs] Update BOTD#4

[altafan]

We must update BOTD#3 by moving to psetv2 (drop in/out blinding keys and add unblinded inputs) and by adding fee asset and amount to the swap request.

It is also very important to describe thoroughly how to use these new fields, in particular how to correctly charge the fees to either amount_p or amount_r.

[altafan]

@tiero shall we just update the spec or we want to add some kind of versioning for having a doc for both the "old" and "new" versions of this BOTD in the dev portal?

[tiero]

@tiero shall we just update the spec or we want to add some kind of versioning for having a doc for both the "old" and "new" versions of this BOTD in the dev portal?

We can simply link to old commit

ie. "If you looking for v1 spec go to this link on Github"

[altafan]

Alright.

Another thing I want to point out is that we added fee_asset and fee_amount to the SwapRequest message but it's hard to give an explanation for them since the current doc refers to Alice and Bob as generic proposer and receiver of a swap.

We should refer to these actors as provider and trader (which are concepts of BOTD#4) to properly explain these 2 new fields of the SwapRequest message.

Maybe it's more appropriate to add these fields to the ProposeTradeRequest message (BOTD#4) since they seem to be related more to the trade context rather than the swap one?

[tiero]

related more to the trade context rather than the swap one

Agreed let's move there