fix(pageserver): add the walreceiver state to tenant timeline GET api endpoint (#5196)

Add a `walreceiver_state` field to `TimelineInfo` (response of `GET /v1/tenant/:tenant_id/timeline/:timeline_id`) and while doing that, refactor out a common `Timeline::walreceiver_state(..)`. No OpenAPI changes, because this is an internal debugging addition.

Fixes #3115.

Co-authored-by: Joonas Koivunen <joonas.koivunen@gmail.com>

pageserver/src/http/routes.rs

@@ -285,6 +285,8 @@ async fn build_timeline_info_common(
     let state = timeline.current_state();
     let remote_consistent_lsn = timeline.get_remote_consistent_lsn().unwrap_or(Lsn(0));
 
+    let walreceiver_status = timeline.walreceiver_status();
+
     let info = TimelineInfo {
         tenant_id: timeline.tenant_id,
         timeline_id: timeline.timeline_id,
@@ -305,6 +307,8 @@ async fn build_timeline_info_common(
         pg_version: timeline.pg_version,
 
         state,
+
+        walreceiver_status,
     };
     Ok(info)
 }