Cool, I like this a lot!

I'll try to rebase this PR soon, so that it's usable by others.

Missing pieces here:
-- signing the assemblies that get packaged (should be easy enough)
-- VersionInfo.cs should file should be link-included in multiple projects (to provide version number)
-- (support compilation in x86 mode - that's that default for .NET 4.5)
-- some files are generated using templates (like VersionInfo.cs) and this hasn't been reflected in this PR yet.
-- NativeDeps.targets is used to copy prebuilt native libraries to the output directory when in development mode (working with gRPC repo itself). This PR doesn't support that yet (mostly because project.json has limited support for such things).

About that last bit, NuGet v3 and dotnet cli now have good support for prebuilt native libraries. No need for special steps anymore, they just need to be in a special folder structure to get picked up.
See https://www.nuget.org/packages/libuv and https://github.com/aspnet/libuv-package.

@ah- thanks for the tip. I was aware that libuv does some interesting tricks (and what we do I inspired by that to some extent), but we also need to make things work properly on .NET 4.5 and with mono.

Also, project.json seems to be getting deprecated, so we might need to approach things differently (I think eventually, the XML-based projects will make things simpler to us, but currently it's unclear what exactly needs to be done).

rebased.

If there's anything outstanding on this that needs to be done, let me know!

@bobbytables see above "missing pieces".

Also, we need to figure out how to address the problem with project.json files getting deprecated.

Do we need to do anything about project.json? Once the nuget package is built it doesn't matter if it came from a project.json project or an old-school msbuild one. The files are the same. So grpc could build with project.json for now until the replacement is in place.

Not sure what to do about VersionInfo.cs though.

Updated this PR with a bunch of improvements:

• Grpc.Core.Tests and Grpc.Examples.Tests are now passing with dotnet5.4
• make old style and new style project files not collide when using VS2015 (used projectname.project.json trick from Ignore PROJECT.JSON when using .CSPROJ dotnet/msbuild#394).
• make code build under dotnet5.4 again
• use the new "copyToOutput" mappings setting to copy native libraries to the right folder when building (currently only for windows, but good as a proof of concept)

Grpc.InteropTesting project is still not compatible with dotnet5.4, the biggest problem being that
some of the dependencies don't support dotnet5.4:

• CommandLineParser
• Moq
• (Google.Apis.Auth might be problematic as well, as the GetApplicationDefaultCredentials code is currently only available for .NET 4.5)

I started skimming for the later project.json files, as basically once you've got one right for each "type" of project (core library, tests, sample) the rest will be the same. Hope it's useful.

I addressed the comments from Jon (thanks!) and added templates for generating the project.json files.

Currently, all the tests are passing under CoreCLR under linux, windows and mac (except the few tests that are disabled using conditional compilation when compiling under netstandard1.5).

At this stage, it seems the PR is worth integrating.

LGTM

tests.tools/buildgen/generate_projects.sh failure in sanity tests is unrelated (seen and fixed on master).