[RFE] Create custom manifest spec for UOR that extends OCI (or the oras artifact spec)

[jpower432]

What is the problem you're trying to solve
The UOR client is storing attributes in OCI annotations. The issue with this solution is that attributes are meant to be typed. Since annotations are a map with string as the value, the option has been to store a JSON formatted doc within a custom UOR annotation.

Describe the solution you'd like
A less brittle solution is to create a spec to extend the OCI specification or (oras Artifact specification) to include a new UOR-specific field for attributes.

Additional context
oras.Pack is used to generate and pack the manifest into the content.Store. This functionality would have to be recreated but using the UOR spec.

[jpower432]

Related to #63