Use blob sha256 to generate taskID instead of download url

[hyy0322]

Feature request:

Use blob sha256 to generate taskID.

func TaskID(url string, meta *commonv1.UrlMeta) string {
	return taskID(url, meta, false)
}

Currently dragonfly use image download url to generate task ID.

Use case:

Two different image share some same blobs. If image repo not the same, the url is not same. So task ID are different.The same cache will missed.

UI Example:

[yxxhero]

make sense

[gaius-qi]

I will consider implementing in the rust client.