Add DbMigrationTest key and update connection logic

Introduced a new static readonly string `DbMigrationTest` in the `Key` class for database migration tests. Updated connection string logic in `Program.cs` to use `DbMigrationTest` when `useDbMigration` is true, allowing for dynamic connection string selection based on environment configuration.

EnvelopeGenerator.Application/Key.cs

@@ -74,4 +74,9 @@ public static class Key
     /// 
     /// </summary>
     public static readonly string Default = nameof(Default);
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public static readonly string DbMigrationTest = nameof(DbMigrationTest);
 }