Expose message ID list from `ReferencesHeader`

Feature or enhancement

Proposal:

When sending, parsing or validating mails which are replies to other mails it is necessary to access the message IDs from the In-Reply-To and References headers. These, however, are currently not exposed.

# Example use case
if "In-Reply-To" in msg and "References" in msg:
    [in_reply_to_token] = msg["In-Reply-To"].message_ids
    [*_, most_recent_reference_token] = msg["References"].message_ids

    if in_reply_to_token.value != most_recent_reference_token.value:
        raise ValueError(f"Error: References list end must match In-Reply-To message ID")

A workaround is to access the private _parse_tree property which does expose the message_id, though like any private property that should obviously be avoided. This is comparable to AddressHeader which already exposes addresses.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Start by reading ReferencesHeader and the existing AddressHeader API, then inspect how ReferencesHeader currently exposes its private _parse_tree data. Make the message IDs available through the public header interface so the shown In-Reply-To and References usage works without private access, and verify that the exposed values preserve the existing message ID information.