Interfaces of Interfaces is invalid GraphQL

[smyrick]

Library Version
1.0.0

Describe the issue
According to the spec, an interface can not implement another interface. This is not caught by our schema generator but instead just returns the first level interface as the type in the schema.

However there is an outstand RFC into the GraphQL spec to allow this: graphql/graphql-spec#373

Until this is approved and updated we should fail with an exception to better inform schema developers of the limitations when an interface has a supertype that is not @GraphQLIgnored

To Reproduce

This test passes, do we want to make it not pass?

import com.expediagroup.graphql.TopLevelObject
import com.expediagroup.graphql.testSchemaConfig
import com.expediagroup.graphql.toSchema
import org.junit.jupiter.api.Test
import kotlin.test.assertEquals
import kotlin.test.assertNotNull
import kotlin.test.assertNull

class InterfaceOfInterfaceTest {

    @Test
    fun `interface of interface`() {
        val queries = listOf(TopLevelObject(InterfaceOfInterfaceQuery()))

        val schema = toSchema(queries = queries, config = testSchemaConfig)

        assertEquals(expected = 1, actual = schema.queryType.fieldDefinitions.size)
        assertEquals(expected = "getClass", actual = schema.queryType.fieldDefinitions.first().name)

        assertNotNull(schema.typeMap["MyClass"])
        assertNotNull(schema.typeMap["SecondLevel"])
        assertNull(schema.typeMap["FirstLevel"])
    }

    interface FirstLevel {
        val id: String
    }

    interface SecondLevel : FirstLevel {
        val name: String
    }

    class MyClass(override val id: String, override val name: String) : SecondLevel

    class InterfaceOfInterfaceQuery {
        fun getClass() = MyClass(id = "1", name = "fooBar")
    }
}

produces

schema {
  query: Query
}

interface SecondLevel {
  id: String!
  name: String!
}

type MyClass implements SecondLevel {
  id: String!
  name: String!
}

type Query {
  getClass: MyClass!
}

[smyrick]

Changing this to an enhancement since we still technically generate a valid schema, just in the Kotlin world we are losing some information