Is your feature request related to a problem or challenge? Please describe what you are trying to do.

User's should be able to specify implementation specific attributes when performing operations on objects within different object store implementations. This is similar to the issue discussed in #330.

Describe the solution you'd like

Expand the Attribute enum to allow for a custom attribute to be added, similar to how the Metadata variant does for metadata attributes.

Describe alternatives you've considered

With AWS for example, you could use the aws-sdk-s3 crate to perform a second operation on the object after its been uploaded, however this does not get the benefit of uploading and applying an attribute within a single upload operation.

Additional context

N/A