Add Ldaptive support to get user login time.

@frett 

I'd like to have written unit tests, but could not do so without support for writing to the LDAP attribute `loginTime`, which this PR will not include. I have, however, tested via system integration.

Note that I'm not, nor have I been in past PR's, providing similar support for spring ldap. Unless otherwise requested, I will most likely continue not to do so, since it seems dependent projects are all opting for use of ldaptive.
we should implement this method with a default value that is returned if the attribute doesn't exist or isn't parsed correctly.

I would also say the login time should be set to null if it parses incorrectly.