Implement public close() method

Because the method is not part of the public API and you need to explicitly dig into the source and  it may lead to subtle bugs in some scenarios. And requires the developer to depend on potentially non-deterministic destruct behavior. 

https://github.com/keboola/ex-email-attachments/pull/20/commits/e4d021789bc5a1e56fb770dea832e5e979fa8798#r201359230