Customizable SPN Service Class for GSSAPI Authentication

The JavaMail specifications do not explicitly define how the SPN service classes should be constructed when using GSSAPI authentication. This omission has led to hardcoded service classes, such as `smtp/host@realm`, across implementations, limiting compatibility with various Kerberos environments.

I propose updating the JavaMail specs to:

* Introduce a property, `mail.smtp.sasl.gssapi.spnserviceclass`, to allow customization of the service class.
* Specify that the default can remain `smtp` (or whatever the implementer was using) if no customization is provided.

### References:
* [Name Formats for Unique SPNs](https://learn.microsoft.com/en-us/windows/win32/ad/name-formats-for-unique-spns)

### Eclipse Angus's Implementation:
```java
// https://github.com/eclipse-ee4j/angus-mail/blob/master/providers/smtp/src/main/java/org/eclipse/angus/mail/smtp/SMTPTransport.java

public class SMTPTransport extends Transport {
private String name = "smtp"; // Name of this protocol
...
saslAuthenticator = (SaslAuthenticator) c.newInstance(
new Object[]{
this,
name, // <-- hardcoded service name
session.getProperties(),
logger,
serviceHost
});
...
}

// https://github.com/eclipse-ee4j/angus-mail/blob/master/providers/smtp/src/main/java/org/eclipse/angus/mail/smtp/SMTPSaslAuthenticator.java

public SMTPSaslAuthenticator(SMTPTransport pr, String name,
Properties props, MailLogger logger, String host) {
...
this.name = name;
...
}
...
Map propsMap = (Map) props;
sc = Sasl.createSaslClient(mechs, authzid, name, host, // <-- hardcoded service name
propsMap, cbh);
...

Review the Jakarta Mail specification sections covering SMTP GSSAPI authentication and compare them with the cited Angus files, providers/smtp/src/main/java/org/eclipse/angus/mail/smtp/SMTPTransport.java and SMTPSaslAuthenticator.java. Define the property and its default behavior precisely, then confirm that the specification clearly describes customizable SPN service classes. No test file is mentioned in the issue.

Written by the indexing model from the issue text.