Add a JSON representation of platform packages for a release to release-notes

[martincostello]

The release notes for specific releases of .NET contain a list of all of the relevant NuGet packages and versions that are associated with it (e.g. 8.0.13's), but this information only appears to be available as Markdown.

It would be advantageous for automated tools to be able to access a JSON representation of this information to avoid the need for manual curation of a list of "what packages are part of the platform" or the need to parse the markdown (example: dependabot/dependabot-core#11127).

As well as some tooling of my own for update automation (martincostello/dotnet-bumper, martincostello/update-dotnet-sdk), other tools such as dependabot and dotnet-outdated/dotnet-outdated could access this data to make themselves more intelligent and/or avoid writing their own solutions to determine them.

I assume that the markdown itself is constructed through some sort of automation, so also generating additional JSON shouldn't be much of an additional burden?

/cc @richlander @slang25 @brettfo

[richlander]

I already want to do this for related reasons. My intent is to automate the generation of release notes and make them much shorter with the other data only be available as JSON.

We were talking about this earlier in the week.