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>

libs/pageserver_api/src/models.rs

@@ -381,6 +381,8 @@ pub struct TimelineInfo {
     pub pg_version: u32,
 
     pub state: TimelineState,
+
+    pub walreceiver_status: String,
 }
 
 #[derive(Debug, Clone, Serialize)]