Add support for schema definition in GraphQL schema language

[ossinkine]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	no
Version/Branch	0.11.11

GraphQL schema document requires schema section which describe root operations.
Right now these operations are specified as config parameter overblog_graphql.definitions.schema.query.
I suggest to add support to describe root operations in GraphQL schema language.

schema {
  query: Query
  mutation: Mutation
}

type Query {
  hero(episode: Episode): Character
  droid(id: ID!): Droid
}

[mcg-web]

hi @ossinkine this could be a nice feature indeed.