support for subscriber creation in trigger function

[VeldaKiara]

Updated:

• the EventDto with RecipientsDto which contains "to": { "subscriberId": "string", "email": "string", "firstName": "string", "lastName": "string", "phone": "string", "avatar": "string", "locale": "string", "data": {} },
• the Event API to get the transmissions.
• test cases to accommodate the changes.