Publish Melanzana to GitHub NuGet feed

[qmfrederik]

I'd like to use Melanzana in one of my projects, and it'd be nice to be able to consume it as NuGet packages.

This PR:

• Adds Nerdbank.GitVersioning, SourceLink, so that running nuget pack on each commit returns a unique version number
• Runs dotnet nuget pack as part of the CI pipeline and publishes the packages as artifacts
• Publishes the NuGet packages to the owner's package feed when building from master

Ideally packages are (frequently) pushed to a NuGet repository which doesn't require authentication (nuget.org or Azure DevOps to name a few), but that can be a follow-up PR.

[filipnavara]

Thanks a lot. I will look at the PR soon and probably wire up CI as well.

(I am working on some redesign of the Mach-O object model / reader / writer to support other use cases than just code signing. It will break some of the public APIs unfortunately. I hope it won't affect the API surface you are going to use but will /cc you on the PR before I push it.)