Add more support for HTTP copying

[jskeet]

Potentially:

• CloudEvent.CopyToHttpRequestMessage
• CloudEvent.CopyToHttpResponseMessage
• HttpContent.ToCloudEvent (may not be terribly useful)
• HttpWebResponse.ToCloudEvent
• CloudEvent.CopyToHttpResponse (ASP.NET Core)

We should look at what the current code for each of those would look like, and consider adding extension methods for them.