Copy blob between versioned buckets should copy blob versions

[thclark]

Feature request

Use Case

I want to move a blob from a versioned bucket to a different place in the same versioned bucket (or to a new versioned bucket). I want to move all generations of that blob, keeping its history consistent.

Perhaps this could be done as an option, preserve_generations=True.

Current state

The copy_blob function copies the latest generation of the blob only.