[federation] _service query should return SDL that contains schema imports

[dariuszkuc]

Library Version
6.0.0-alpha.5

Describe the bug
Federation v2 _service { sdl } query should contain imported element information, i.e. it should contain schema type with applied @link directives.

To Reproduce
Run example federated service and execute _service { sdl } query

Expected behavior
SDL returned from the _service { sdl } query contains schema type with applied @link directives

[dariuszkuc]

Blocked until new version of graphql-java is released. Current version of graphql-java does not print schema directives, fixed in graphql-java/graphql-java#2856 but is not yet released.