Add support for customization of the login module password hash mechanism

A generic extension to add a DigestCallback extension interface like the following should be made to the jboss login modules:

public interface DigestCallback
{
/** Pass through access to the login module options */
public void init(java.util.Map options);
/** Pre-hash callout to allow for content before the password */
public void preDigest(java.security.MessageDigest digest);
/** Post-hash callout afer the password has been added to allow for content after the password */
public void postDigest(java.security.MessageDigest digest);
}

options: the option maps passed to the login module with any unknown options that affect the DigestCallback simply passed through
digest: the standard MessageDigest created based on the digest algorithm specified via the login module options.