proxy: store role not found in cache (#6439)

## Problem

There are a lot of responses with 404 role not found error, which are
not getting cached in proxy.

## Summary of changes

If there was returned an empty secret but with the project_id, store it
in cache.

proxy/src/auth/backend.rs

@@ -202,21 +202,18 @@ async fn auth_quirks(
     if !check_peer_addr_is_in_list(&ctx.peer_addr, &allowed_ips) {
         return Err(auth::AuthError::ip_address_not_allowed());
     }
-    let maybe_secret = api.get_role_secret(ctx, &info).await?;
+    let cached_secret = api.get_role_secret(ctx, &info).await?;
 
-    let cached_secret = maybe_secret.unwrap_or_else(|| {
+    let secret = cached_secret.value.clone().unwrap_or_else(|| {
         // If we don't have an authentication secret, we mock one to
         // prevent malicious probing (possible due to missing protocol steps).
         // This mocked secret will never lead to successful authentication.
         info!("authentication info not found, mocking it");
-        Cached::new_uncached(AuthSecret::Scram(scram::ServerSecret::mock(
-            &info.user,
-            rand::random(),
-        )))
+        AuthSecret::Scram(scram::ServerSecret::mock(&info.user, rand::random()))
     });
     match authenticate_with_secret(
         ctx,
-        cached_secret.value.clone(),
+        secret,
         info,
         client,
         unauthenticated_password,