Support overwriting endpoint GraphQL schema

[n1ru4l]

Allow overwriting of the GraphQL schema used by conductor in front of a GraphQL API.
Introspection and Validation should be done against the schema SDL provided, while validated requests should be forwarded to the actual endpoint (requests that contain __schema introspection fields and "normal" query selections, must be properly split and re-attached by the conductor).

Example Config:

config:
  endpoint: "https://my-source.com/graphql"
  schema: "https://kashdlasdkasjdlkq.eg/schema.graphqls" # THIS IS THE GATEWAY SCHEMA
id: "my-source"
type: "graphql" 

Example Endpoint Schema (config.endpoint)

type Query {
  a: Int!
  b: Int!
  foo: String!
}

Example "Schema"/Gateway Schema (config.schema)

type Query {
  foo: String!
}

--> We only want to expose the Query.foo field on the Gateway.

The use case for this is to support schema contracts for single schemas in Hive, see graphql-hive/platform#3856

[dotansimha]

The main blocker for this one is: #437

Also related: #6