Add action to automatically update .NET Core SDK compatibility table

[sophiewigmore]

Summary

Helps resolve #223. Moved from paketo-buildpacks/github-config#351

This code is largely copied over from the previous iteration of this automation that was in the update-dotnet-sdks-and-compat-table Concourse task. It has been converted into a Github Action.

It takes in a buildpack.toml and a new SDK version, and replaces and updates the compatibility table to include the mapped runtime/SDK versions. Almost all of the functionality and logic is the same as it was when we used the Concourse task. The major difference is that the associated runtime version is no longer an input (we now look it up on .NET Core's official release page), and the release JSON is now looked up instead of being passed in as a file.

This will be consumed by a workflow in #256

Use Cases

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[ForestEckhardt]

I am happy with how this looks!