Provide format-specific "container" conversions #231
Description
(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