Create OpenAPI schema registry for @loopback/rest

[kjdelisle]

depends on #567

Overview

We should create a registry that allows the definition of schema

Proposed Acceptance Criteria

Create a schema registry for the application that defines

• response schemas for operations
  • return types
  • response codes
  • etc
• @response decorators to associate this metadata with response functions
  • if the @response has been defined on a function
    • sequence should leverage this data on the app schema to define correct return codes/error classes
    • possible hooks for validating returned object definition and content-type format?
• request schemas for incoming data?

[bajtos]

@kjdelisle I am not sure, is this something we need for MVP?

[stale]

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