refactor: Validate or add @defer directive in schema

[calvincestari]

Closes apollographql/apollo-ios#3287.

The change here is to:

1. Validate any @defer directive that is in the schema document. The validation is to ensure that any schema definition matches the experimental definition of the directive in graphql-js. We do this because some server implementations allow @defer to be used in other locations, such as FIELD, and Apollo iOS codegen does not support those definitions. If an unsupported @defer directive definition is found a warning will be output and the definition will be automatically replaced with a version that we do support. This could result in operation validation failing if the directive was being used in a way that does not match the replaced definition.
2. Automatically add a @defer directive if it is not found in the schema. This mimics the eventual state of the defer proposal where the directive is available to be used without explicit definition.

[calvincestari]

@AnthonyMDev this is ready for another review, should be all done.