Declarative cache: allow specifying lists of key arguments

[martinbonnin]

Use case

When using the declarative cache, it's possible to specify key arguments with @fieldPolicy:

extend type Query @fieldPolicy(forField: "book", keyArgs: "id")

While this works for fields that return a single object, this doesn't work for fields that return a list (or list of lists, etc..):

type Query {
  # Doesn't work 
  books(ids: [String!]!) @fieldPolicy(forField: "book", keyArgs: "ids")
}

Describe the solution you'd like

Not 100% sure yet. This will certainly have to cooperate with pagination.

[BoD]

With #155, simple cases are now handled.