Add ability to migrate from MySQL and SQLite with EF (#4614)

BTCPayServer.Abstractions/Configuration/DataDirectories.cs

@@ -1,3 +1,5 @@
+using System.IO;
+
 namespace BTCPayServer.Configuration
 {
     public class DataDirectories
@@ -7,5 +9,12 @@ namespace BTCPayServer.Configuration
         public string TempStorageDir { get; set; }
         public string StorageDir { get; set; }
         public string TempDir { get; set; }
+
+        public string ToDatadirFullPath(string path)
+        {
+            if (Path.IsPathRooted(path))
+                return path;
+            return Path.Combine(DataDir, path);
+        }
     }
 }