[RFR] Add filters to all* queries

[fzaninotto]

• Generate filter type for entities
• Make filters work
• Fix tests
• Document it

The filter logic was implemented in the resolvers, but not declared in the type system - so it was unusable.

Now, for every type, we add a filter type.

e.g.

type Post {
  id: ID!
  title: String!
  nb_views: Int
}
type PostFilter {
  q: String
  id: ID
  title: String
  nb_views: Int
}

I also modified the type of the filter parameter to match the related filter type:

type Query {
  allPosts(filter: PostFilter): [Post]
}

And now the filter system works:

{
  allCustomers(filter: { first_name: "Agnes" }) {
    first_name
    last_name
  }
}

{
  "data": {
    "allCustomers": [
      {
        "first_name": "Agnes",
        "last_name": "Mathis"
      },
      {
        "first_name": "Agnes",
        "last_name": "Tucker"
      },
      {
        "first_name": "Agnes",
        "last_name": "Pope"
      }
    ]
  }
}

[fzaninotto]

Switching to RFR