Add a C# SDK for .NET core

The recent work on the portability framework makes creating new SDKs easier. It would be really nice if we support C# as a language to write Beam pipelines. C# is still one of the most popular programming languages and it has been historically a poor citizen of the ‘Big Data’ ecosystem.

There is caveat at this moment (April 2018): gRPC [does not support]([https://github.com/grpc/grpc/issues/12159)] yet .NET core 2 so a first version should address .NET core 1 first.

Imported from Jira [BEAM-4010](https://issues.apache.org/jira/browse/BEAM-4010). Original Jira may contain additional context.
Reported by: iemejia.