Improves package download and caching

Adds package caching to reduce download frequency.

Introduces a `PackageDownloadService` with memory caching to store downloaded packages,
identified by mission ID, for reuse. Uses concurrent locks to prevent race conditions during download.

Modifies the worker client service to optionally delete the package after sending,
allowing cached packages to be retained.

src/LiquidCode.Tester.Gateway/Services/WorkerClientService.cs

@@ -19,7 +19,7 @@ public class WorkerClientService : IWorkerClientService
         _configuration = configuration;
     }
 
-    public async Task SendToWorkerAsync(SubmitForTesterModel submit, string packagePath)
+    public async Task SendToWorkerAsync(SubmitForTesterModel submit, string packagePath, bool deletePackageAfterSend)
     {
         var workerUrl = GetWorkerUrlForLanguage(submit.Language);
         _logger.LogInformation("Sending submit {SubmitId} to worker at {WorkerUrl}", submit.Id, workerUrl);
@@ -56,17 +56,20 @@ public class WorkerClientService : IWorkerClientService
         }
         finally
         {
-            // Clean up downloaded package
-            try
+            if (deletePackageAfterSend)
             {
-                if (File.Exists(packagePath))
+                // Clean up package file when it is not needed anymore
+                try
                 {
-                    File.Delete(packagePath);
+                    if (File.Exists(packagePath))
+                    {
+                        File.Delete(packagePath);
+                    }
+                }
+                catch (Exception ex)
+                {
+                    _logger.LogWarning(ex, "Failed to delete package file {Path}", packagePath);
                 }
-            }
-            catch (Exception ex)
-            {
-                _logger.LogWarning(ex, "Failed to delete package file {Path}", packagePath);
             }
         }
     }