phpstan: add vcr plugin for stricter checks

[dbu]

as per discusssion we should add a composer require the vcr plugin in a first step of the phpstan job, to have it validate correctly. the php tests would discover if we messed up with validation.

some nice to haves would be to have the configuration class error when configuring without vcr without having the plugin installed, and https://github.com/php-http/HttplugBundle/blob/b7ad2c1951edec36ef0bd9897e309b0038c59a54/src/DependencyInjection/HttplugExtension.php should throw an exception at the beginning of the method if vcr is not available.