Add channel_leave message push type

[fussybeaver]

Due to error:

\"unknown variant `channel_leave`, expected one of `bot_message`, `me_message`, `channel_join`, `bot_add`, `bot_remove`, `channel_topic`, `channel_purpose`, `channel_name`, `message_changed`, `message_deleted`, `thread_broadcast`, `tombstone`, `joiner_notification`, `slackbot_response`\"

In payload:

{
  "envelope_id": "7070764d-a47c-4969-9638-64dcd9dbb832",
  "payload": {
    "token": "hRcCJba8Je3CQu4LmsDNH5Ww",
    "team_id": "_redacted_",
    "api_app_id": "_redacted_",
    "event": {
      "type": "message",
      "subtype": "channel_leave",
      "ts": "1649770793.940659",
      "user": "UMVCS7CDU",
      "text": "<@UMVCS7CDU> has left the channel",
      "channel": "C033G46KA8L",
      "event_ts": "1649770793.940659",
      "channel_type": "channel"
    },
    "type": "event_callback",
    "event_id": "Ev03ARCB2PD5",
    "event_time": 1649770793,
    "authorizations": [
      {
        "enterprise_id": null,
        "team_id": "_redacted_",
        "user_id": "U02SNDCEZB5",
        "is_bot": true,
        "is_enterprise_install": false
      }
    ],
    "is_ext_shared_channel": false,
    "event_context": "_redacted_"
  },
  "type": "events_api",
  "accepts_response_payload": false,
  "retry_attempt": 2,
  "retry_reason": "timeout"
}

Unsure if I need to add any serialization structs, but doesn't look like it at first glance..

[abdolence]

LGTM 👍🏻