Adds streaming generation that returns a streaming

- Introducues generate_content_stream that returns a Tokio Steam instead
of a Queue. This allows using the standard stream APIs from
tokio-streams.
- Replace future-utils with tokio-streams, mainly due to better
ergonomics for using the filter_map stream combinator.

src/token_provider.rs

@@ -7,7 +7,7 @@ pub trait TokenProvider {
         -> impl std::future::Future<Output = Result<String>> + Send;
 }
 
-impl<'a> TokenProvider for Arc<dyn gcp_auth::TokenProvider + 'a> {
+impl TokenProvider for Arc<dyn gcp_auth::TokenProvider> {
     async fn get_token(&self, scope: &[&str]) -> Result<String> {
         let token = self.token(scope).await;
         match token {