rest: Support HTTPS and pre-instantiated HTTP/S instances

[kjdelisle]

Overview

Developers obviously want to use https with their applications (and well they should!). Currently, the alpha implementation of @loopback/rest has no support whatsoever for this concept!

Acceptance Criteria

• Add configuration support in RestComponent/RestServer for provision of:
  • Certificates/key materials
  • https configuration options (rejectUnauthorized, requestCert, etc...)
• Add configuration support to accept an existing http, https or https/2 server instance.

[bajtos]

Labelled as non-MVP - users can get HTTPS support by puting an Nginx instance in front of their LoopBack app, or use a sidecar like Envoy when running a microservice-based architecture like http://istio.io/

[dhmlau]

Talked to @raymondfeng , it might be solved through #1082

[dhmlau]

Closing as duplicate of #430