Add `shutdown` method to `Destination` trait

[psteinroe]

Feature Request

I created a little project using etl in which the Destination is not as stateless as it was initially designed to be. To properly cleanup resources on shutdown, I would be great to add stop or shutdown method to the trait that is called by the pipeline when it is shutting down.

For context, my destination creates a COPY stream very similar to the initial table sync of etl during recovery. It would be great to pass a proper shutdown signal to interrupt the stream if needed.

It also runs a daily async maintenance task which is super quick but should be awaited during shutdown too.

Happy to open the PR myself once the schema change work is merged.

Thanks for all the great work! It's super fun to work with the library.

[iambriccardo]

Thanks for reporting! This is a nice addition that we will add in the future!