Function graphql_handler 

pub async fn graphql_handler<A, Q, M, S>(
    schema: Extension<Schema<Q, M, S>>,
    headers: HeaderMap,
    req: GraphQLRequest,
) -> GraphQLResponse
where
    A: Send + Sync + 'static,
    Q: ObjectType + Send + Sync + 'static,
    M: ObjectType + ContainerType + Send + Sync + 'static,
    S: SubscriptionType + Send + Sync + 'static,