graphql: finish subscriptions

api/graphql/gqlgen.yml

api/graphql/graph/operation.generated.go

api/graphql/graph/root_.generated.go

api/graphql/graph/subscription.generated.go

api/graphql/models/gen_models.go

api/graphql/resolvers/subscription.go

api/graphql/schema/operation.graphql

api/graphql/schema/subscription.graphql

cache/events.go

cache/multi_repo_cache.go

cache/repo_cache.go

cache/repo_cache_test.go

cache/subcache.go