[SPIKE] Create @loopback/boot to support declarative JSON/YAML files

[raymondfeng]

Staging goals:

1. Be able to load LB 3.x artifacts including models/datasources
2. Support LB next artifacts, such as model/datasource/repository/api-spec/...

Story

As a LoopBack developer, I'd like to declare my artifacts as JSON or YAML files so that I can continue enjoying the declarative experience.

Acceptance Criteria

• On application boot:
  • Read each of the directories in src for their corresponding artifacts (models, datasources, controllers, etc)
  • For each artifact:
    • Parse the artifact as an object
    • Semantically validate that the object meets the requirements of the artifact type it specifies
    • Wire up the artifact to the application context using the appropriate artifact prefix

Example

Given a src directory:

src/
  datasources/
    foo.datasource.json

we'll want to parse the datasource artifact, which contains:

{
    "host": "127.0.0.1",
    "port": 3001,
    // etc
}

Once we've loaded the file, we'll want to ensure that it has all of the properties required for it to be a datasource (in this case, a "host" property might be required), and then bind it to the context.

const datasource = validateDatasource(dsObj);
app.bind(`datasources.${name}`).to(datasource);

Rules

• If more than one artifact exists with a different extension (foo.model.json and foo.model.yaml)
  • prefer JSON over YAML
  • if there is a TS file and JSON/YAML, merge the JSON/YAML definition into the parsed TS class
    foo.model.json

Questions to Answer

• How will we merge programmatic models (TypeScript classes) with declarative forms?
• What artifacts will we support for declarative use?
  • models
  • datasources
  • ACL
  • middleware
  • remoting
  • boot scripts
  • repositories
    • If we're supporting them in declarative form, what is the schema for that?
• What will the schema be for each of these artifacts?

[bajtos]

Cross-posting from #533:

We should prescribe a convention for organizing different kinds of files in LoopBack-Next projects:

• controllers
• sequence actions
• models and repositories
• value providers
• etc.