Hook up the AI-to-AI trade rate

We have a [trade factor](https://github.com/C7-Game/Prototype/blob/8b7643a1641ac43ca25ec0d6b1efc09df16c4d78/C7Engine/AI/PlayerAI.cs#L395-L399) for AI and human trades, but it is hardcoded. We should be using the value from the [difficulty](https://github.com/C7-Game/Prototype/blob/8b7643a1641ac43ca25ec0d6b1efc09df16c4d78/C7Engine/C7GameData/Difficulty.cs#L36).