Added integration with [zvonok.com](https://zvonok.com) service.
Features:
- Phone number validation
- Test calls
- Selection of pre-recorded audio
- Making calls
- Processing call status
- Acknowledgment alert group (optional)
To process the call status, it is required to add a postback with the
GET method on the side of the zvonok.com service with the following
format ([more info
here](https://zvonok.com/ru-ru/guide/guide_postback/)):
```${ONCALL_BASE_URL}/zvonok/call_status_events?campaign_id={ct_campaign_id}&call_id={ct_call_id}&status={ct_status}&user_choice={ct_user_choice}```
The names of the transmitted parameters can be redefined through environment variables.
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Co-authored-by: Innokentii Konstantinov <innokenty.konstantinov@grafana.com>
