Add static APIs to a new nuget package

[TomFinley]

In #1930 we introduced new static assemblies. Yeah! But then we forgot to add the new assemblies to any nuget package. Whoops. Which means they're effectively unshipped.

The ultimate solution should be that there are separate nugets for the static API. However, until that happens and we decide what shape those nugets would take, for the sake of the v0.9 we should maintain the status quo, which was that the new projects introduced in #1930 are shipped with their old assemblies.

/cc @eerhardt @shauheen

[TomFinley]

Actually, no we can't do that, because #1930 put all the static extensions into a single assembly, no matter what, so you can't use any of the static API without including everything. This is quite altogether wrong, but complicates the story, and I don't see a reasonable way of teasing it out. We would need to ship a nuget temporarily, which is too bitter of a pill. So maybe we just don't ship static API for v0.9. I don't see a way to include it under the current circumstances.

[eerhardt]

@TomFinley - IMO it doesn't make sense to do the work to split the static API into a separate assembly, but then keep that same assembly in the core Microsoft.ML NuGet package.

Can we just create a new NuGet package named Microsoft.ML.StaticPipe (the same name as the assembly) instead? It isn't that much work (see below). Also, if we decide to change the static pipe packages later - it wouldn't affect anyone who uses the core Microsoft.ML nuget. Whereas, if we leave it in Microsoft.ML, changing it later could affect any of our customers, since they all use that package.

1. Create a new folder Microsoft.ML.StaticPipe under https://github.com/dotnet/machinelearning/tree/master/pkg
2. Create 2 files basically copied from something like https://github.com/dotnet/machinelearning/tree/master/pkg/Microsoft.ML.Onnx
   1. Microsoft.ML.StaticPipe\Microsoft.ML.StaticPipe.nupkg
   2. Microsoft.ML.StaticPipe\Microsoft.ML.StaticPipe.symbols.nupkg
3. Edit the Microsoft.ML.StaticPipe.nupkg file as necessary.
4. Add <IncludeInPackage>Microsoft.ML.StaticPipe</IncludeInPackage> to https://github.com/dotnet/machinelearning/blob/master/src/Microsoft.ML.StaticPipe/Microsoft.ML.StaticPipe.csproj

[TomFinley]

Hmmm, all right Eric, let's do that.