Represent support for publishing messages with `headers` and `references` in `extras`

[QuintinWillison]

As pointed out by @lmars in #3 (comment) "Publish message with headers in extras" and "Publish message with references in extras".

Relevant spec points include: TM2i, TP3i, RSL6a2.

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-2783