Allow building upkeep to be modified using effects

[XHawk87]

Is your feature request related to a problem? Please describe.
Currently it's not possible to directly modify the upkeep costs of city improvements. The specific case that prompted this was that the help text for Fundamentalism indicated that the upkeep for happiness buildings is eliminated, but this isn't the case. It is offset instead by tithes, i.e. Happiness_To_Gold. However, it could be useful in many cases to be able to modify the upkeep costs of buildings depending on various factors.

Describe the solution you'd like
Add a Building_Upkeep effect that adds or subtracts upkeep from buildings by the given value.

Describe alternatives you've considered
The janky workaround we have is to use Output_Add_Tile to offset the cost, however multipliers from marketplace etc are added on top of this, which isn't exactly what we want.

Additional context
Relates to longturn/LTT-LTX#24
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