Closes #8894: Add first and last name username user api select

[minitriga]

Fixes: #8894

[minitriga]

Updated. Cheers for the review.

Just to note that get_full_name will return first and last name individually if the other is not set.

For example if the first_name is not defined it will only show surname.

>>> User.objects.get(pk=2).first_name
''
>>> User.objects.get(pk=2).last_name
'Gittings'
>>> User.objects.get(pk=2).get_full_name()
'Gittings'