fix(proxy/http): fix HTTP/1 client idle timeouts (#4004)

When constructing the HTTP/1 client, we configure connection pooling, but
notably do not provide a timer implementation to Hyper. This causes hyper's
connection pool to be configured without idle timeouts, which may lead to
resource leaks, especially for clients that communicate with many virtual hosts.

This change updates the HTTP/1 client builder to use a Tokio timer, which allows
Hyper to manage idle timeouts correctly.

Author: olix0r

linkerd/proxy/http/src/h1.rs

@@ -127,6 +127,7 @@ where
                     hyper_util::client::legacy::Client::builder(TokioExecutor::new())
                         .pool_max_idle_per_host(self.pool.max_idle)
                         .pool_idle_timeout(self.pool.idle_timeout)
+                        .pool_timer(hyper_util::rt::TokioTimer::default())
                         .set_host(use_absolute_form)
                         .build(HyperConnect::new(
                             self.connect.clone(),