[compute] fix: background worker that collects installed extension metrics now updates collection interval (#12277)

## Problem

Previously, the background worker that collects the list of installed
extensions across DBs had a timeout set to 1 hour. This cause a problem
with computes that had a `suspend_timeout` > 1 hour as this collection
was treated as activity, preventing compute shutdown.

Issue: https://github.com/neondatabase/cloud/issues/30147

## Summary of changes

Passing the `suspend_timeout` as part of the `ComputeSpec` so that any
updates to this are taken into account by the background worker and
updates its collection interval.

libs/compute_api/src/spec.rs

@@ -184,6 +184,11 @@ pub struct ComputeSpec {
     /// Download LFC state from endpoint_storage and pass it to Postgres on startup
     #[serde(default)]
     pub autoprewarm: bool,
+
+    /// Suspend timeout in seconds.
+    ///
+    /// We use this value to derive other values, such as the installed extensions metric.
+    pub suspend_timeout_seconds: i64,
 }
 
 /// Feature flag to signal `compute_ctl` to enable certain experimental functionality.

libs/compute_api/tests/cluster_spec.json

@@ -3,6 +3,7 @@
 
     "timestamp": "2021-05-23T18:25:43.511Z",
     "operation_uuid": "0f657b36-4b0f-4a2d-9c2e-1dcd615e7d8b",
+    "suspend_timeout_seconds": 3600,
 
     "cluster": {
         "cluster_id": "test-cluster-42",