Add support for marking specific tags as "Featured" in the manifest

[lbussell]

There is a desire to have a more detailed and/or dynamic "Featured Tags" section in the https://github.com/dotnet/dotnet-docker repo's README files.

Currently those are written by hand to refer to floating tags.

I think that instead, we should be able to mark a tag as "Featured" and then generate the featured tags list/table during README generation.

For example:

        // ...
        {
          "productVersion": "$(dotnet|8.0|product-version)",
          "sharedTags": {
            "$(dotnet|8.0|fixed-tag)-bookworm-slim": {},
            "$(dotnet|8.0|minor-tag)-bookworm-slim": {},
            "$(dotnet|8.0|fixed-tag)": {
              "featured": true
            },
            "$(dotnet|8.0|minor-tag)": {
              "featured": true
            }
          },
          "platforms": [
            ...
          ]
        },
        // ...

[lbussell]

[Triage] While this is a good solution to keeping featured tags up to date, it provides little value on its own. It'll be easier to prioritize this if it's in the context of a larger proposal for README UX improvements.