Added database entities, started with node crud

MoonlightServers.ApiServer/Database/Entities/ServerVariable.cs

@@ -0,0 +1,11 @@
+namespace MoonlightServers.ApiServer.Database.Entities;
+
+public class ServerVariable
+{
+    public int Id { get; set; }
+
+    public string Key { get; set; }
+    public string Value { get; set; }
+
+    public Server Server { get; set; }
+}