proxy: delay auth on retry (#4929)

## Problem

When an endpoint is shutting down, it can take a few seconds. Currently
when starting a new compute, this causes an "endpoint is in transition"
error. We need to add delays before retrying to ensure that we allow
time for the endpoint to shutdown properly.

## Summary of changes

Adds a delay before retrying in auth. connect_to_compute already has
this delay

proxy/src/auth/backend/classic.rs

@@ -5,7 +5,7 @@ use crate::{
     auth::{self, AuthFlow, ClientCredentials},
     compute,
     console::{self, AuthInfo, CachedNodeInfo, ConsoleReqExtra},
-    proxy::handle_try_wake,
+    proxy::{handle_try_wake, retry_after},
     sasl, scram,
     stream::PqStream,
 };
@@ -62,10 +62,13 @@ pub(super) async fn authenticate(
             }
             Ok(ControlFlow::Continue(e)) => {
                 warn!(error = ?e, num_retries, retriable = true, "couldn't wake compute node");
-                num_retries += 1;
             }
             Ok(ControlFlow::Break(n)) => break n,
         }
+
+        let wait_duration = retry_after(num_retries);
+        num_retries += 1;
+        tokio::time::sleep(wait_duration).await;
     };
     if let Some(keys) = scram_keys {
         use tokio_postgres::config::AuthKeys;