Implement query builder

[KashubaK]

Borrow some inspiration from mst-gql's query builder, I quite like it:

useQuery(AllMessagesQuery, (data) => data.messages(message => message.content.createdAt.author(user => user.name)))

• [x] Implement model selector generation
• [x] Build GQL selections with generated list of keys
• [x] Integrate into Mutation and Query classes