Update a 2-tier for the embassy building.

Now tier-1 is much cheaper to build, only diplomacy buffs, and can recruit most good units (but not the best units of each faction and no agent).

Tier-2 is same as before.

Balance update:

1. Significantly reduced initial cost to build embassy, but add in big per-turn cost.
2. The campaign buffs are stronger to compensate the per-turn cost.
3. Also restricted embassy building in special cities only.

The purpose is to prevent embassy spamming, making it a balanced structure economically, and possible to build and maintain one embassy early to mid game.

Previously embassy was too OP economically. With agent spamming and no per-turn cost, the economy boost was simply huge after built.

Let me know any feedback.