HTTP endpoint factory

[bajtos]

Rework the way how an HTTP server is setup and configured for LoopBack applications (RestServer instances) following the approach outlined in #1082.

IMPORTANT

The spike was based on an assumption that we will support different frameworks (Express, Koa, etc.). Based on the spike outcome, we decided to go forward with a single framework and use Express request/response types everywhere. As a result, most of the spike code needs to be simplified by removing generic type arguments etc.

https://github.com/strongloop/loopback-next/blob/2bddc47ccda56a50c52a8a92facd87e3a3f4944d/packages/http-server/src/common.ts#L12-L48

https://github.com/strongloop/loopback-next/blob/2bddc47ccda56a50c52a8a92facd87e3a3f4944d/packages/http-server/src/common.ts#L116-L212

Acceptance criteria

• There should be a new module for creating HTTP and HTTPS servers.
• @loopback/rest should use this new module to start/stop the server
  - The module should support HTTP config only

Out of scope

The following features are out of scope, but please keep them in mind when designing the new module.

• HTTPS support - see Support HTTPS out of the box #430
• Configurable framework factory function - see Make the factory HTTP framework function configurable #1290

[dhmlau]

Closing this as PR #1369 has merged