Single file transfer (#26)

* move to single file transfer for server

* fix cache get

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Co-authored-by: rootdarkarchon <root.darkarchon@outlook.com>

MareSynchronosServer/MareSynchronosStaticFilesServer/Utils/UserRequest.cs

@@ -1,3 +1,3 @@
 namespace MareSynchronosStaticFilesServer.Utils;
 
-public record UserRequest(Guid RequestId, string User, string FileId);
+public record UserRequest(Guid RequestId, string User, List<string> FileIds);