Edited field in the app mention event

[zhshishkin]

This commit add edited field to the app mention event

Sometimes it is important to react on the mention only once. The field helps to distinguish whether the message is original or edited

#961

Example:

{
    "envelope_id": "********",
    "payload": {
        "token": "********",
        "team_id": "T********",
        "api_app_id": "A********",
        "event": {
            "client_msg_id": "********",
            "type": "app_mention",
            "text": "********",
            "user": "U********",
            "ts": "1706785424.099119",
            "blocks": [
                {
                    "type": "rich_text",
                    "block_id": "7Kvco",
                    "elements": [
                        {
                            "type": "rich_text_section",
                            "elements": [
                                {
                                    "type": "text",
                                    "text": "text"
                                }
                            ]
                        }
                    ]
                }
            ],
            "team": "T********",
            "edited": {
                "user": "U********",
                "ts": "1706785427.000000"
            },
            "thread_ts": "1706785424.099119",
            "reply_count": 1,
            "reply_users_count": 1,
            "latest_reply": "1706785424.915589",
            "reply_users": [
                "U********"
            ],
            "is_locked": false,
            "subscribed": false,
            "channel": "C********",
            "event_ts": "1706785424.099119"
        },
        "type": "event_callback",
        "event_id": "********",
        "event_time": 1706785424,
        "authorizations": [
            {
                "enterprise_id": null,
                "team_id": "T********",
                "user_id": "U********",
                "is_bot": true,
                "is_enterprise_install": false
            }
        ],
        "is_ext_shared_channel": false,
        "event_context": "********"
    },
    "type": "events_api",
    "accepts_response_payload": false,
    "retry_attempt": 0,
    "retry_reason": ""
}

[zhshishkin]

Hi, @parsley42, could I get this pr reviewed please?

[parsley42]

Trivial