Add C# SDK

[cnunciato]

Adds C# to the Buildkite SDK family. 🎉

using Buildkite.Sdk;

var pipeline = new Pipeline();

pipeline.AddStep(new CommandStep
{
    Label = "some-label",
    Command = "echo 'Hello, world!'"
});

Console.WriteLine(pipeline.ToJson())

$ npx nx run app-csharp:run
{
  "steps": [
    {
      "command": "echo 'Hello, world!'",
      "label": "some-label"
    }
  ]
}

(cc @zchase)