Support implementation-specific attributes

**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`](https://docs.rs/object_store/latest/object_store/enum.Attribute.html) 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](https://docs.rs/aws-sdk-s3/latest/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](https://docs.aws.amazon.com/AmazonS3/latest/API/API_PutObject.html#API_PutObject_RequestSyntax) operation.

**Additional context**

N/A

Start with the Attribute enum linked in the issue and review the discussion in #330, then compare the existing Metadata variant with the object-store implementations. The work is complete when callers can provide implementation-specific attributes during a single upload, including the AWS use case; the issue names no files or tests to run.

Written by the indexing model from the issue text.