* 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

Use docker for csharp, java_compatibility, and python_compatibility. Remove prepare_build_linux_rc that is no longer needed.