proxy small tweaks (#6398)

## Problem

In https://github.com/neondatabase/neon/pull/6283 I did a couple changes
that weren't directly related to the goal of extracting the state
machine, so I'm putting them here

## Summary of changes

- move postgres vs console provider into another enum
- reduce error cases for link auth
- slightly refactor link flow

proxy/src/console/provider.rs

@@ -248,23 +248,75 @@ pub trait Api {
     async fn get_role_secret(
         &self,
         ctx: &mut RequestMonitoring,
-        creds: &ComputeUserInfo,
+        user_info: &ComputeUserInfo,
     ) -> Result<Option<CachedRoleSecret>, errors::GetAuthInfoError>;
 
     async fn get_allowed_ips(
         &self,
         ctx: &mut RequestMonitoring,
-        creds: &ComputeUserInfo,
+        user_info: &ComputeUserInfo,
     ) -> Result<CachedAllowedIps, errors::GetAuthInfoError>;
 
     /// Wake up the compute node and return the corresponding connection info.
     async fn wake_compute(
         &self,
         ctx: &mut RequestMonitoring,
-        creds: &ComputeUserInfo,
+        user_info: &ComputeUserInfo,
     ) -> Result<CachedNodeInfo, errors::WakeComputeError>;
 }
 
+#[derive(Clone)]
+pub enum ConsoleBackend {
+    /// Current Cloud API (V2).
+    Console(neon::Api),
+    /// Local mock of Cloud API (V2).
+    #[cfg(feature = "testing")]
+    Postgres(mock::Api),
+}
+
+#[async_trait]
+impl Api for ConsoleBackend {
+    async fn get_role_secret(
+        &self,
+        ctx: &mut RequestMonitoring,
+        user_info: &ComputeUserInfo,
+    ) -> Result<Option<CachedRoleSecret>, errors::GetAuthInfoError> {
+        use ConsoleBackend::*;
+        match self {
+            Console(api) => api.get_role_secret(ctx, user_info).await,
+            #[cfg(feature = "testing")]
+            Postgres(api) => api.get_role_secret(ctx, user_info).await,
+        }
+    }
+
+    async fn get_allowed_ips(
+        &self,
+        ctx: &mut RequestMonitoring,
+        user_info: &ComputeUserInfo,
+    ) -> Result<CachedAllowedIps, errors::GetAuthInfoError> {
+        use ConsoleBackend::*;
+        match self {
+            Console(api) => api.get_allowed_ips(ctx, user_info).await,
+            #[cfg(feature = "testing")]
+            Postgres(api) => api.get_allowed_ips(ctx, user_info).await,
+        }
+    }
+
+    async fn wake_compute(
+        &self,
+        ctx: &mut RequestMonitoring,
+        user_info: &ComputeUserInfo,
+    ) -> Result<CachedNodeInfo, errors::WakeComputeError> {
+        use ConsoleBackend::*;
+
+        match self {
+            Console(api) => api.wake_compute(ctx, user_info).await,
+            #[cfg(feature = "testing")]
+            Postgres(api) => api.wake_compute(ctx, user_info).await,
+        }
+    }
+}
+
 /// Various caches for [`console`](super).
 pub struct ApiCaches {
     /// Cache for the `wake_compute` API method.