[Feature]: Implement PackageBaseAddress/6.12.0 for downloading the README

[jgonz120]

Related Problem

Rendering README for packages in PMUI

The Elevator Pitch

Implement a new resource type PackageBaseAddress/6.12.0. This will indicate to clients that the server has implemented the ability to download the README using the following pattern {@id}/{LOWER_ID}/{LOWER_VERSION}/readme. An example of this url would be https://api.nuget.org/v3-flatcontainer/newtonsoft.json/13.0.3/readme.

Additional Context and Details

While we update the protocol on NuGet.org, we can update the comment to mention that readme exists there.