Add C# namespace

[maxreb]

I would like to see the option for a C# namespace, so underneath the java option something this:

option csharp_namespace = "Chirpstack.Api.As";

Unfortunately, we are not able to define a base namespace in C# and Api.DeviceService is not a really good, distinguishable name for it, I think. Every time there is a new version of your API I add it with a tool, but it would be quite nice if I could keep the files as they are, and I am certain that people using C# will benefit from it.

I can do it on my own and make a PR if it's wanted.
Also, I could create a sample C# client if wanted

[brocaar]

A PR to add support for generating C# code is more than welcome :-)