🐛 [firestore-send-email] Support DSN (SMTP NOTIFY) option in Firestore Send Email extension

### [READ] Step 1: Are you in the right place?

Yes, this issue is about a feature request for a specific extension in this repository.

### [REQUIRED] Step 2: Extension name

This feature request is for extension: **firestore-send-email**

### What feature would you like to see?

The extension currently allows passing basic mail fields (`to`, `message`, `headers`, etc.) into Nodemailer, but does not expose Nodemailer's **DSN (Delivery Status Notification)** option.

I would like to be able to include a `dsn` object in the Firestore document to request delivery status notifications via SMTP (`RCPT TO NOTIFY=SUCCESS,FAILURE,DELAY`). This would allow server-level delivery reports and is more reliable than non-standard headers like `Return-Receipt-To`.

Example Firestore document:

```json
{
"to": ["recipient@example.com"],
"message": {
"subject": "Test DSN",
"text": "Hello with DSN"
},
"dsn": {
"id": "12345",
"return": "headers",
"notify": ["success", "failure", "delay"],
"recipient": "sender@example.com"
}
}