copy_blob: Allow setting object metadata

[remko]

The JSON API copy operation allows providing object metadata on copy. This avoids having to do multiple operations on an object to set metadata such as content-type, cache-control, ... on a copied object.

This is currently not exposed in the Python SDK on copy_blob (copy_blob never sends a body in its request). Could this be exposed?

[cojenco]

Thanks for the feature request. I will work with the team to have this prioritized.