Represent support for JSON serialisation of objects

[QuintinWillison]

As pointed out by @lmars in #3 (comment) "Publish JSON serializable object", we're perhaps not clear enough that 'plain objects' can be (semi-)transparently encoded to and decoded from JSON in our SDKs for message data.

Relevant terminology from the features spec:

• JsonObject or JsonArray like object
• objects capable of JSON representation
• JSON Message payload
• stringified as JSON Object or Array
• JSON-encodable

Especially relevant are: RSL4c, RSL4d, TM2d, TP3e.

This is discrete and different from our support for JSON or MessagePack in the protocol.

Work on this issue includes deciding whether new feature node(s) are required, or whether this capability can be included or better defined within the scope of an existing feature node or nodes.

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-2749