Allow specifying source IP for outgoing emails in SMTP module

Problem/Motivation

My server has multiple network interfaces, each with its own IP address.
Due to the hosting provider's security policy, outgoing connections to SMTP (port 587) are only allowed from a specific IP address — not from the default one.

Currently, Drupal does not provide a way to specify which local IP address should be used for outgoing SMTP connections.
I need the ability to configure Drupal to send emails via SMTP using a specific source IP.

Proposed resolution

The SMTP module is built on top of the PHPMailer library (https://github.com/PHPMailer/PHPMailer), which has the ability to set options such as:

$mail->SMTPOptions = [
  'socket' => [
    'bindto' => '217.253.72.159:0',
  ],
];

The proposed resolution is to add a new configuration option in the module form to specify the outgoing source IP, and inject this value into the corresponding PHPMailer settings.

Remaining tasks

1. Add a new "Outgoing IP Address" field to the SMTP module configuration form.
2. Use the value of this field when configuring PHPMailer for outgoing emails.

User interface changes

A new text field will be added to the SMTP module configuration form, allowing the administrator to specify the outgoing source IP address for SMTP connections.