Rationalize NameIDFormat config. (#1741)

This was a mixture of legacy supported options, some of them
deprecated, and data types, and also implemented differently
in some places than others.

This changes it once more but hopefully for the better:
* The value is always an array or null/unset.
* You can set it to a specific array to get a specific policy.
* You can leave it unset/null to keep the default policy.
* You can set it to the empty array to signal that you do not
want any policy to be sent.
* The string and bool types are no longer allowed.

All in all this means that we can make code much simpler, a lot
less if-branching, and also typewise make use of correct typehints.

The behaviour changes are as follows:
- We drop the already deprecated option to set it as a string
  (deprecated in 1.17).
- To not send the element you need to change false to [];
  this was not deprecated before but I believe setting it to
  false was already broken in master.