Encode and decode the `Data`-and-JSON-valued properties

[lawrence-forooghian]

It's not yet clear what is the meaning of the Binary-valued ObjectMessage-related properties in the spec, namely:

• ObjectData.bytes
• ObjectOperation.initialValue

These clearly can't always be binary over the wire, because JSON doesn't support that. I've held off on implementing these properties for now, until we get clarification about what they mean, which I believe will be coming in the upcoming LiveObjects spec.

Update: OK, this is now specified by ably/specification#335; implement it.

┆Issue is synchronized with this Jira Task by Unito