Add RC1 Support

[akashsinghal]

• Updates artifact spec to RC1
• Implements sorting
• Implements filtering
• Implements registry level OCI discover
• Implements repository level OCI discover
• Implements pagination
• Adds tests for artifact manifest handler
• Adds tests for referrers API

References #4

[akashsinghal]

@aviral26 thanks for the review! I have addressed your comments:

• I added media type validations
• It starts from page 1 by default if it cannot find any matching manifests provided in the nextToken
• The nextToken takes in a comma separated list of digests. The Link header generated returns the last 3 referrer manifest digests in the nextToken field. (I have also set the minimum page size to 3 to accommodate 3 digests being returned. If n < 3 is provided, it defaults to the default page size of 50).
• Added a new ErrorMalformedNextToken error. It is thrown if the nextToken parsing fails at the list and individual digest level

[estebanreyl]

I've been giving this some thought myself, but we may want to introduce some way to register errors from the extension API itself.

[aviral26]

Just looking for some context on the changes to this file.

1. Previously, both repository and registry routes were returned but now it seems to be one of the two?
2. Why are extensions added only if endpoints exist - when would that not be the case?

[akashsinghal]

1. Yes that's correct. So previously, both routes were returned but the subset of routes we want to return depends on if the discover request is at the registry or repository level. The distinction is now being made at the EnumerateRegistered level. If the Repository field in the context is nil, then we know that we are operating at the registry level and only want to return registry level routes for that extension namespace.
2. It's possible that for an extension namespace, that there doesn't exist any routes at the scoped registry or repository level. In that case, we wouldn't want to add the enumerateExtension object to the list since no endpoints were added. The referrers extension would be an example: referrers only operates at the repository level but if we are enumerating at the registry level, then the scopedRoutes would be empty and we wouldn't want to add to the enumeratedExtensions list.

[aviral26]

This now may also contain a whole lot of annotations. Do we want to limit the maximum size of the returned response?

[akashsinghal]

I have removed the annotations from the referrers response since the referrers spec sample response does not contain the field. There's also a concern that a large number of annotations can cause response size issues.

[akashsinghal]

Follow up PR to add GC and more extensions unit tests will be made to the rc1 branch