[RFC][SPIKE] feat(graphql): add a PoC for graphql integration

[raymondfeng]

Spiking on GraphQL integration using type-graphql.

High-level mapping:

• Resolver -> Controller
• *Type -> Model
• Field -> Property
• fieldResolver -> Relation

High-level design:

• Add GraphQLServer to expose graphql endpoints
• Add graphql related decorators for controllers
• Add/unify graphql decorations for models/properties

Maybe we should also create a graphql connector.

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

[bajtos]

I'd like to put this pull request on hold until we figure out how to approach experimental features - see #2676

[raymondfeng]

Move to strongloop/loopback-labs#3