Add `ObjectData.json` and remove `ObjectData.encoding`

[VeskeR]

We have changed how JSON-encodable data is sent over the wire, see ably-js implementation for reference ably/ably-js#2052.

Also see related DR: https://ably.atlassian.net/wiki/x/a4CK_g, and internal slack discussion: https://ably-real-time.slack.com/archives/C07D55B6NLQ/p1752572643408459