Change CD to make pre-releases available

[mbhoek]

As a devops, I want to have the ability to build 'pre-release' versions of the package.

Currently we always release what is commited to main (adopting the Mainline paradigm of gitversion). This was in line with the expectation that the plugin was stable and only minor changes would be made for each . release of SpecFlow.

However we now wish to rearchitect major parts of the plugin, with several contributions (e.g. #74, #75, #12, #77), so now there is a need to be able to release intermediary version, i.e. 'pre-releases'.

Plan:

• change gitversion mode to Continuous Delivery
• change CD so that main get build, signed, pushed to NuGet as pre-release
• change CD so that 'tags' get build, signed, pushed to NuGet as release
• NOTE: CD currently actively sets the version tag, this must change to passively reading the tag Correct, GitVersion is source
• TODO: who will set the tag?
• Code Signing does not work anymore; fix and document