Add CI/CD pipeline for NuGet packaging and deployment

[aslotte]

Background

We need to automate the build and deployment of our nuget packages.
Each package needs to have a consistent build number.

We should probably run the dotnet pack command with versioning after each CI build (we can have different versions for PR builds vs master builds).

The deployment to nuget.org should most likely be a separate workflow file as it should be manually triggered once we have a release.

[aslotte]

This article may be a good starting point:
https://cloudblogs.microsoft.com/industry-blog/en-gb/technetuk/2019/06/18/perfecting-continuous-delivery-of-nuget-packages-for-azure-artifacts/

[aslotte]

This is also a good one:
https://intellitect.com/azure-devops-nugets/

[aslotte]

It may make sense that our local PR builds push to GitHub Packages and our release builds goes to nuget.org