(Re-)introduce release builds with Azure Pipelines (#2176)

Since stewardship of the project has transitioned back to Microsoft from
GitHub, we're moving the official release builds from GitHub Actions to
Azure Pipelines. This is mainly a requirement to enable Microsoft
code-signing of executables, packages, etc.

The release pipeline will also publish a GitHub release and the .NET
Tool packages to NuGet.org.

The GitHub Actions release workflow is no longer in use (it also
required access to approve releases from internal GitHub code-signing
systems, which we no longer have access to).

Author: mjcheetham