Offchain signatures preparation

[zmrocze]

Severity	CVSS	Vulnerability Type
Low	0.0	other

Description

This is a small remark on a challenge that will arise in the future.
Namely, when building a transaction the builder expects the number of signatures that will be signing the transaction. This is needed to calculate the fee. The consequence is that the offchain code defining the transaction before it is send to the owners for signatures will have to know the number of signatures that are to be included - and this is including the signatures for paying the fee.

In practice the easiest is to always use exactly threshold many signatures.