Extension: GraphQL

This issue is tracking progress on implementing native GraphQL layer making it easy to build GraphQL APIs in LoopBack.

- Package name: [`@loopback/graphql`](https://www.npmjs.com/package/@loopback/graphql)
- Example app: [`@loopback/example-graphql`](https://github.com/strongloop/loopback-next/tree/master/examples/graphql)
- Docs: [Creating GraphQL APIs](https://loopback.io/doc/en/lb4/GraphQL.html)

## Progress

Stability: ⚠️Experimental⚠️

- [x] #5545 added the initial implementation of native GraphQL layer, built using [type-graphql](https://github.com/MichalLytek/type-graphql).
- [x] #6313 added support for GraphQL subscriptions
- [x] #6313 improved graphql configuration and subscriptions

## What's next

We don't have any specific plans now, we are looking for your feedback! Please give the new extension a try and let us know what works for you well and what is missing.

Start with the @loopback/graphql package, the @loopback/example-graphql app, and the Creating GraphQL APIs documentation; review the initial implementation and subscription work linked in #5545 and #6313. The issue has no specific next task or acceptance criteria, so feedback on what is missing would need to define what done means.

Written by the indexing model from the issue text.