Support descriptions for docs on object fields of a model

[minox86]

We need a way to specify a description of every field of an object in order to enrich its formal specifications (OpenAPI, GraphQL, etc.).

We can already do it for primitive types as following:

const User = () => model.entity({
   id: model.string({ description: "The ID must be a valid GUID." }),
})

but we can't do it for referenced types:

const User = model.entity({ ... })
const Post = () => model.entity({
      author: User, // ???
 })

A suggested syntax to keep the locality of the description to the field and considering technical concerns is the following:

const User = model.entity({ ... })
const Post = () => model.entity({
      author: model.asField(User, { description: "The user who wrote the post." }),
 })

[edobrb]

Final syntax:

export const User = () =>
  model.entity(
    {
      id: idType,
      firstName: model.string(),
      lastName: model.string(),
      email: model.email(),
      posts: model.array(Post),
      givenLikes: model.array(Like),
      followers: model.describe(model.array(User), 'Users that follows me'),
      followeds: model.describe(model.array(User), 'Users followed by me'),
      registeredAt: model.datetime(),
      loginAt: model.datetime(),
    },
    { description: 'User of the system' },
  )