Find union members and interface implementors from @JsonSubTypes #862

Follow-up to #200 and #854. #854 only helps when the class behind a union or interface is sealed. For plain hierarchies you still need a dictionary entry per member.

A lot of these classes already list their subtypes for Jackson:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "__typename")
@JsonSubTypes(
    JsonSubTypes.Type(value = MemberA::class, name = "MemberA"),
    JsonSubTypes.Type(value = MemberB::class, name = "MemberB"))
interface AnyMember

@briankrug's patch reads that annotation in SchemaClassScanner and matches each subtype to an object type by JsonSubTypes.Type#name.

Main issue is, the JSON type name isn't the GraphQL type name. It defaults to "", and it makes a serialization annotation part of the schema mapping. Probably safer to use @JsonSubTypes only as the list of candidates and match on the class simpleName, same as the sealed lookup. It's the only route for non-sealed classes on Java 11 though.