Create annotated reference assemblies nuget packages

[manfred-brands]

I noticed that on large solutions, with 100s of project, that the annotator creates reference packages in every project taking up considerable amount of diskspace.

So I created a tiny project, target every framework and run it once. Then I packed the resulting sets of annotated dlls into a nuget package per framework. Now we reference these and save on diskspace. Currently published to our corporate nuget server.

I had expected to safe on time as well, but your tool is efficient that on a fast SSD it doesn't seem to make much difference.

If you think this could be a solution for your caching problem, I can create a PR here.

I don't know if we are allowed by MS to publish these annotated packages on NuGet or why MS hasn't created annotated reference packages themselves to aid in take up of nullable reference types.