Add a minimal GraphQL API

.rubocop.yml

Gemfile

Gemfile.lock

app/controllers/graphql_controller.rb

app/graphql/gitlab_schema.rb

app/graphql/loaders/base_loader.rb

app/graphql/loaders/full_path_loader.rb

app/graphql/loaders/iid_loader.rb

app/graphql/mutations/.keep

app/graphql/types/merge_request_type.rb

app/graphql/types/mutation_type.rb

app/graphql/types/project_type.rb

app/graphql/types/query_type.rb

app/graphql/types/time_type.rb

config/dependency_decisions.yml

config/routes/api.rb

lib/gitlab/graphql/authorize.rb

spec/controllers/graphql_controller_spec.rb

spec/graphql/gitlab_schema_spec.rb

spec/graphql/loaders/full_path_loader_spec.rb