feat: support directives with the union annotation

[bherrmann2]

📝 Description

the @GraphQLUnion annotation does not support directives. This is to add support by allowing it to be a meta-annotation where you can create an annotation for your union that you then annotate with @GraphQLUnion and directives

// Defined in some other library
class SharedModel(val foo: String)

// Our code
class ServiceModel(val bar: String)

@SomeDirective
@GraphQLUnion(
    name = "CustomUnion",
    possibleTypes = [SharedModel::class, ServiceModel::class],
    description = "Return one or the other model"
)
annotation class CustomUnion

class Query {

    @CustomUnion
    fun getModel(): Any = ServiceModel("abc")
}

🔗 Related Issues

[valentyn-arkho]

Great job 👍