Provide format-specific "container" conversions

[jskeet]

(From #230)

Various formats (certainly JSON, XML and Protobuf) have a natural container format (e.g. JObject/JsonElement, XElement, the CloudEvent message) and it can be useful to expose conversions for those containers rather than require serialization to/from opaque binary data.

We should:

• Implement this for the JSON formatters (it's already present in Protobuf)
• Document a pattern
• Implement it in the XML formatter if it's not already