[Security][Login Link] Allow null and DateTime objects to be used as signatureProperties

[wouterj]

Q	A
Branch?	5.x
Bug fix?	no
New feature?	yes
Deprecations?	no
Tickets	-
License	MIT
Doc PR	-

Returning DateTime objects seems like a common use-case to automatically expire all login links when one is used or to only allow the login link to be used once.

Before

class User
{
    private ?\DateTime $lastAuthenticatedAt = null;
    // ...

    public function getLastAuthenticatedAtString(): string
    {
        return null === $this->lastAuthenticatedAt ? '' : $this->lastAuthenticatedAt->format('c');
    }
}

security:
  firewalls:
    main:
      login_link:
        # ...
        signature_properties: ['lastAuthenticatedAtString']

After

class User
{
    private ?\DateTime $lastAuthenticatedAt = null;
    // ...

    public function getLastAuthenticatedAt(): ?\DateTime
    {
        return $this->lastAuthenticatedAt;
    }
}

security:
  firewalls:
    main:
      login_link:
        # ...
        signature_properties: ['lastAuthenticatedAt']

---

The disadvantage of this patch is that there needs to be some boundary as to which objects we want to support casting to a scalar, but I'm convinced that DateTime objects will commonly be used as signature properties.

cc @weaverryan

[fabpot]

Thank you @wouterj.