Add Support for Custom Headers in SMTP Appender

Currently, the [SMTP Appender](https://logging.apache.org/log4j/2.x/manual/appenders/network.html#SmtpAppender) does not support adding custom headers to outgoing email messages. In contrast, the [HTTP Appender](https://logging.apache.org/log4j/2.x/manual/appenders/network.html#HttpAppender) allows users to define custom headers using `` elements. Introducing similar support for the SMTP Appender would enhance feature parity across appenders and improve user experience and flexibility.

### Proposed Solution

Both the HTTP and SMTP Appenders extend `AbstractFilterable` and already support `` configuration elements. In the HTTP Appender, these `` elements are interpreted as HTTP headers. Similarly, the SMTP Appender could treat `` entries as custom email headers.

**Example:**

```xml

```

This change would preserve existing behavior while enabling greater customization for advanced email use cases.