Add a C# namespace option to the proto
Add a C# namespace option to the proto

The naming choice is based on the C# SDK core being
"CloudNative.CloudEvents". This creates a bias towards using the C#
SDK instead of "roll-your-own" - but I think that's okay,
personally. Happy to consider other options.

Signed-off-by: Jon Skeet <8a9602d66334021c18cbf0bdb2fa15f4cb6de9df@google.com>

cloudevents/spec