Extract router from core into @loopback/rest

[kjdelisle]

Overview

I had a discussion with @bajtos and @virkt25 about the plan for HTTP/S and GRPC support, which led us to conclude that we should break out the implementation of the router to ensure that:

• Our beta design is flexible enough to allow arbitrary mounting of protocol components
• We don't need to have the gRPC implementation done in time for beta (because it'll be a component!)

Acceptance Criteria

• Extract router from @loopback/core and into its own package (tentative name is @loopback/rest, and related classes
  • Sequence
  • Sequence actions
  • Routing decorators
  • Open API decorators
  • Controllers

Controllers

We need to remove app.controller for now, as it's implementation-specific (OpenAPI), and then have the @loopback/rest component provide its own* sugar function for binding controllers. This will let other protocol implementations decide how these things are built on a case-by-case basis.