Fix gradle plugin for correct generation federated schema

[RoMiRoSSaN]

The current plugin does not fully fulfill its role. This is a small fix for him. If you have the opportunity to check the correctness of its operation, I would be grateful. Unfortunately, this is the only option that worked as it should.

[jmartisk]

Thanks @RoMiRoSSaN , from a quick glance it makes sense, I'll give it some more proper testing as soon as I get some time

[RoMiRoSSaN]

@jmartisk I added autodetect of federation directives. Please, check it