Components 'loaded' Promise & loopback <--> components contract

[anodynos]

New Feature:

It would be great if we could add support for a Promise to be returned for each loopback-component-XXX loaded status + more.

This will allow us to decide whether to perform a task or not, or even disable parts of the API's functionality, wait before running tests etc.

See for example this workaround beginning of a nice and simple loopback <--> components contract for auto-migration snowyu/loopback-component-auto-migrate.js#4

In summary, we need a (lightweight) contract so components can be looked up as app.components['superPlugin'], and provide some rudimentary lifecycle (then) hooks (eg loaded, stopped etc) and even have a custom exported object.

[bajtos]

@raymondfeng @davidcheung could you PTAL please? How does this fit into the new loopback-boot design/architecture?

FWIW, loopback-component-explorer is "publishing" its configuration via app.set('loopback-component-explorer'), see https://github.com/strongloop/loopback-component-explorer/blob/188603f352e1a6a6ba7587aaa0a9e3e1a75ce387/index.js#L38