loopback-next-hello-extension sample package

[Setogit]

purpose: example code come along with extension blog post: task #524
refactor loopback-next-hello-world so that the two work seamlessly.

[bajtos]

In this sprint, as part of #525, @virkt25 will be working on loopback-next-hello-extension project aiming at new extension developers.

I think this issue is mostly a duplicate of #525, but let's keep it open in case the blog post have different needs and some tweaking of loopback-next-hello-extension is needed.

[dhmlau]

closing as duplicate of #525.
cc @virkt25