feat(v2/objectstore): return closer when opening bucket avoiding leaks

gocloud's blob.Bucket.Close is a no-op for GCS, meaning the storage.Client is never closed. We may run into similar concerns with upcoming additional providers.

This MR adjusts our Provider Open function to return an io.Closer, that Client.Shutdown closes alongside the bucket.

I'd like the review to strongly consider if we need this. If we expect buckets to be a long living handle, opened at startup and kept until the process ends, this may not be needed - as the leak would not accumulate in normal use.