allow alliance extension Fixes #491 (#1314)

## Description:

About 30s before an alliance is about to expire, both players receive a
prompt to extend the alliance. If both players agree the alliance is
extended.

## 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
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

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

evan

src/core/execution/ExecutionManager.ts

@@ -2,6 +2,7 @@ import { Execution, Game } from "../game/Game";
 import { PseudoRandom } from "../PseudoRandom";
 import { ClientID, GameID, Intent, Turn } from "../Schemas";
 import { simpleHash } from "../Util";
+import { AllianceExtensionExecution } from "./alliance/AllianceExtensionExecution";
 import { AllianceRequestExecution } from "./alliance/AllianceRequestExecution";
 import { AllianceRequestReplyExecution } from "./alliance/AllianceRequestReplyExecution";
 import { BreakAllianceExecution } from "./alliance/BreakAllianceExecution";
@@ -111,6 +112,10 @@ export class Executor {
           this.mg.ref(intent.x, intent.y),
           intent.unit,
         );
+      case "allianceExtension": {
+        return new AllianceExtensionExecution(player, intent.recipient);
+      }
+
       case "upgrade_structure":
         return new UpgradeStructureExecution(player, intent.unitId);
       case "create_station":