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.
for now return null if the time can't be parsed. If we have need to specify a different default value down the line we can add in the second function to handle different default values.