[API Explorer] - Add static file serving capabilities to RestServer

[jannyHou]

Story

As a LoopBack developer, I would like to have my RestServer provide static file hosting so that I can retrieve front-end content (HTML/CSS/JS).

(Additional note: This functionality would be required for an API Explorer Component)

Acceptance Criteria

• Add a new API to both RestServer and RestApplication to allow app developers to mount static middleware, for example app.static(path, options). Leverage Express middleware server-static to implement the actual file serving.
  • Should allow a user to specify a directory and tweak serve-static options
  • Should set the Content-Type header of the response based on the file's MIME type.
• Add a short section/guide to our documentation explaining users how to use this new API.

Stretch goals:

• Should return a nicely formatted "404" error if the content could not be found.
  Comment by @bajtos: Ideally, a LB4 app should have only one error handler responsible for handling all errors regardless of their source (controller methods, static middleware, input argument validation, etc.) To get there, we need to rethink and improve the error handling story in general, see The error response for a request with malformed JSON body provides too little information #1434. Unless we can find a way how to get nice 404 errors for static content with very little effort, I am proposing to defer this item and make it a part of The error response for a request with malformed JSON body provides too little information #1434.

Removed from scope by @bajtos on 2018-06-25:

• Should default to /src/public
• Should recursively map target directory's file and folder paths in memory (NOT THE CONTENT)

[bajtos]

We are targeting API server developers for MVP release. Serving static files (e.g. single-page applications) is out of scope of that. We should revisit this after MVP and decide what different kinds of servers we want to support in LoopBack 4+

[virkt25]

Useful links to check out:
https://github.com/expressjs/serve-static
https://www.npmjs.com/package/mime