Support DateTimeImmutable

[tmarsteel]

In several places the Code assumes that \DateTime is being used. That is a problem for me and i would like the bundle to support \DateTimeImmutable. From what i've seen by searching for DateTime its only a few places to change so i will not do a fork & PR for this:

Based off of v2.0.1 (4f92bfb):

• Change the Signatures of UserInterface::setLastLogin and UserInterface::setPasswordRequestedAt (and implementation in User accordingly) to accept \DateTimeInterface instead of \DateTime.
• In User.php line 479 change the instanceof check from \DateTime to \DateTimeInterface

I can currently not reset the password of my users because the password reset date is a DateTimeImmutable. That is caused by that instanceof check. I can work around this pretty easily but i still think that code with as much usage as this bundle should not have that kind of bug. You have to, at the very least, throw an exception near that instanceof so that people know what the issue is and dont have to dig into the code.

On the Background of this feature request:

I use \DateTimeImmutable exclusively because the mutability of \DateTime has caused a great deal of trouble for me in the past.