Method to authenticate a {@code Subject} (phase 1).
The implementation of this method authenticates a {@code Subject}. For example, it may prompt for {@code Subject} information suchas a username and password and then attempt to verify the password. This method saves the result of the authentication attempt as private state within the LoginModule.
@exception LoginException if the authentication fails
@return true if the authentication succeeded, or false if this{@code LoginModule} should be ignored.