Add support to be able to register extensions

[dmikusa]

CNB extensions are a new a cool technology. They may not be quite as reusable as buildpacks (they can be more tied to a particular builder/base image), but they can definitely be reused with some constraints. This means the registry should support them so that we can allow users to discover new extensions.

If paired with #119, this would also allow us to build some graphs between the builders and extensions to show how they are being used (i.e. this extensions is used by 5 builders, click to see a list) and to show known compatibilities.