Needs to be made to work with Symfony 5

[boite]

symfony/security, on which this client lib depends, is now four separate packages which makes it practically impossible for this lib to work in a Symfony 5 app.

It may be necessary to make other changes too; see the upgrade guide.

Maybe we should branch or fork this lib for Symfony 4.4 and 5 apps or maybe we can do some magic and split the security bits of this lib from the client bits.