[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.

control_plane/src/endpoint.rs

@@ -780,6 +780,7 @@ impl Endpoint {
                 endpoint_storage_addr: Some(endpoint_storage_addr),
                 endpoint_storage_token: Some(endpoint_storage_token),
                 autoprewarm: false,
+                suspend_timeout_seconds: -1, // Only used in neon_local.
             };
 
             // this strange code is needed to support respec() in tests