* Bump target frameworks from netcoreapp1.0 to netcoreapp2.2.
Move global.json up to root of repo, change SDK ver to 2.2.100
Change .net core sdk in dockerfile for kokoro to ver 2.2.100
* Re-add curl install
* Change all exe target to 2.1
* Fix incorrect versions in global.json and Dockerfile
* Downgrade version to 2.1 to match exe targets
* introduce separate testing Dockerfile for C#
* revert changes to the shared Dockerfile
* use netcoreapp2.1 for C# conformance tests
* use language specific dockerfile for testing C#
* Edit compatibility tests script to use parameters instead of file copies
* install dotnet SDK on windows before running the tests
* update csharp_EXTRA_DIST
