Support for managed JAX-RS sub resources

[BrianSetz]

Smallrye Open API can handle normal JAX-RS sub resources:

@Path("resource")
class Resource {
    @Path("sub-resource")
    public SubResource subResource() {
        return new SubResource();
    }
}

class SubResource {
    @GET
    public String subResource() {
        return "GET sub resource";
    }
}

However, there is no support for managed sub resource locators.

@Path("resource")
class Resource {
    @Path("managed-sub-resource")
    public Class<ManagedSubResource> managedSubResource() {
        return ManagedSubResource.class;
    }
}

@RequestScoped
class ManagedSubResource {
    @Inject
    MyService myService;

    @GET
    public String managedSubResource() {
        return myService.callSomething();
    }
}

The key difference is that instead of returning a new instance of a class, only the class itself is returned. We use managed sub resources extensively so we don't have to inject at the parent resource and then pass the injected object to the sub resource constructor, and so we can let CDI manage the lifetime.

I know where to fix this and will submit a draft PR, but I will need some help with the correct implementation and the best place to introduce tests.