Provide lifecycle information about the SDK

There are simple and good ways to determine which runtimes exist and what there support status. In particular, it is easy to write tools that treat runtime versions as a first class concept. I'm in the process of doing just that. I am realizing that it is very difficult to reason about SDK feature bands in the same way. It is something that I'm going to need to solve for the tool to do the right thing.

There are two key (example) questions:

- Which SDK feature bands exist for 8.0?
- Which of those feature bands are actively supported?

For example, should I upgrade to the latest 8.0.2xx build due to a recent runtime CVE or is 8.0.2xx EOL and I need to move to a later feature band? Trolling through `releases.json` files is not an acceptable answer to that.

SDK feature bands have always been weird. The fact that they are effectively undocumented is contributing to that.

Here's the file that provides a great experience for runtime versions: https://github.com/dotnet/core/blob/main/release-notes/releases-index.json.