pageserver: add list API for LocationConf (#6329)

## Problem

The `/v1/tenant` listing API only applies to attached tenants.

For an external service to implement a global reconciliation of its list
of shards vs. what's on the pageserver, we need a full view of what's in
TenantManager, including secondary tenant locations, and InProgress
locations.

Dependency of https://github.com/neondatabase/neon/pull/6251

## Summary of changes

- Add methods to Tenant and SecondaryTenant to reconstruct the
LocationConf used to create them.
- Add `GET /v1/location_config` API

pageserver/client/src/mgmt_api.rs

@@ -209,6 +209,15 @@ impl Client {
         Ok(())
     }
 
+    pub async fn list_location_config(&self) -> Result<LocationConfigListResponse> {
+        let path = format!("{}/v1/location_config", self.mgmt_api_endpoint);
+        self.request(Method::GET, &path, ())
+            .await?
+            .json()
+            .await
+            .map_err(Error::ReceiveBody)
+    }
+
     pub async fn timeline_create(
         &self,
         tenant_id: TenantId,