[C#][Flight] Add beginning on flight code for net core

Adding support for flight in net core would be a required component for me, since a project I am working on is looking to migrate from our proprietary protocol to Arrow.

This is something I can start myself to do, but I would like some feedback, firstly:

Allot of the C# code is internal only, and at the start now I added the grpc libraries in the "Apache.Arrow" project, this requires a bump of the supported framework version from 1.3 to 1.5

If this is not ok, a lot of the existing code would have to be refactored.

The gRPC client framework requires .netstandard 2.1, I plan to add this to a seperate project completely, to not remove support from netstandard1.5 and above from the main lib.

Implementation wise I am looking mostly on the java code to try and not reinvent the wheel. But an initial version would not come with the buffer optimizations that java and C++ has if that is ok.