Only sign IOV transactions that have an explicit fee payer set

[webmaster128]

The fee payer in BNS is optional and in the backend the payer falls back to the first signature of the transaction. Since signatures have no fixed order, everyone who signs a transaction can potentially become the fee payer. In order to avoid surprises, the client should only sign transactions with an explicit fee payer set.

I have no idea how exactly to implement as it will probably be hard to integrate this idea into our stack.