Generates a signed authentication token for the specified user.
Application-specific properties may be added to the token.
This method returns the token that will be sent in the response when
AuthenticationHandler sends its authentication response
if getErrorData() returns null.
If this method is called multiple times, only the last authentication token is returned to the user.
expiration - The expiration information for the authentication token.
customData - (Optional). Custom application-defined properties for the authentication token.
The authentication token which will be sent to the user.