Turnstile: require token before joining a multiplayer game (#2572)

When user tries to join either a public or private multiplayer game, the
turnstile callback is triggered, and the turnstile token is passed to
the server when joining a game.

## 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:

evan

src/core/configuration/DefaultConfig.ts

@@ -80,6 +80,10 @@ const numPlayersConfig = {
 } as const satisfies Record<GameMapType, [number, number, number]>;
 
 export abstract class DefaultServerConfig implements ServerConfig {
+  turnstileSecretKey(): string {
+    return process.env.TURNSTILE_SECRET_KEY ?? "";
+  }
+  abstract turnstileSiteKey(): string;
   allowedFlares(): string[] | undefined {
     return;
   }