move reqwest client out of thread spawning

this way, we only spawn one, and reuse that.

plume-common/src/activity_pub/mod.rs

@@ -152,11 +152,11 @@ where
         headers.insert("Digest", request::Digest::digest(&body));
         let sig = request::signature(sender, &headers)
             .expect("activity_pub::broadcast: request signature error");
-        rt.spawn(async move {
         let client = ClientBuilder::new()
             .connect_timeout(std::time::Duration::from_secs(5))
             .build()
             .expect("Can't build client");
+        rt.spawn(async move {
             client
                 .post(&inbox)
                 .headers(headers.clone())