Error for non-bean classes annotated with @GraphQLApi

[datho7561]

According to the spec, only CDI beans can be annotated with @GraphQLApi. However, the rules on the @GraphQLApi annotation are not fine-grained enough to specify this, so the java compiler and language server don't report this as an error.

Trying this out with smallrye-graphql with a Quarkus project, if you try to put the annotation on a non-bean class, the application won't start up. Only non-abstract classes (i.e. not interface, not enums) actually support the annotation.

It would be nice to add an error if the user puts the @GraphQLApi on a type that doesn't support it.