Before, we were limited to either all inputs having defined gas or none at all.
This commit adds more flexibility, such as enabling one address to cover gas costs
for others.

First we keep the same exact computing of gas per input, so we could potentially
reduce the selected gas as previously.

But on the second phase, in `updateMissingFeeFromSelectedGas`
we check how many is already paid by the addresses that defined some gas.

There are now two cases:
1. The explicit gas doesn't cover the overall gas needed for the tx:
   The addresses that didn't defined some gas will share this missing
   gas
2. There is enough gas for the all tx, it means we can update the gas
   to 0 to addresses that didn't define a gas.

…y-gas