Trade causes Pollution

[XHawk87]

Is your feature request related to a problem? Please describe.
Currently, it isn't very realistic that trade has no effect on pollution generation. After the industrial revolution, most economic work was driven by the burning of fossil fuels.

Describe the solution you'd like
Add a Pollu_Trade_Pct effect than can be customised by ruleset that works like Pollu_Prod_Pct and adds to the pollution score alongside it. Default 0%.

Describe alternatives you've considered
We could add a MinTrade requirement, and use it to modify Pollu_Pop_Pct to make this based on population within certain trade bounds, but that seems like an awkward solution.

Additional context
Improving GW Mechanics on discord.