Record train trading stats (#2891)

## Description:

The current gold stats don’t include gold generated by trains, even
though this is a significant part of the economy for many players.

This PR tracks those stats with two values:
- other players trains visits the player station
- the player trains visits any station

Linked to this infra PR: https://github.com/openfrontio/infra/pull/242

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

tests/core/game/TrainStation.test.ts

@@ -22,6 +22,10 @@ describe("TrainStation", () => {
       }),
       addUpdate: vi.fn(),
       addExecution: vi.fn(),
+      stats: vi.fn().mockReturnValue({
+        trainExternalTrade: vi.fn(),
+        trainSelfTrade: vi.fn(),
+      }),
     } as any;
 
     player = {