add SystemInfoService, increase API to 3

MareSynchronosServer/MareSynchronos.API/SystemInfoDto.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace MareSynchronos.API
+{
+    public record SystemInfoDto
+    {
+        public double CpuUsage { get; set; }
+        public long CacheUsage { get; set; }
+        public int UploadedFiles { get; set; }
+        public double NetworkIn { get; set; }
+        public double NetworkOut { get; set; }
+        public int OnlineUsers { get; set; }
+        public long RAMUsage { get; set; }
+    }
+}