TenantInfo: expose generation number (#6348)

Generally useful when debugging / troubleshooting.

I found this useful when manually duplicating a tenant from a script[^1]
where I can't use `neon_fixtures.Pageserver.tenant_attach`'s automatic
integration with the neon_local's attachment_service.

[^1]: https://github.com/neondatabase/neon/pull/6349

pageserver/src/disk_usage_eviction_task.rs

@@ -633,7 +633,7 @@ async fn collect_eviction_candidates(
 
     let mut candidates = Vec::new();
 
-    for (tenant_id, _state) in &tenants {
+    for (tenant_id, _state, _gen) in &tenants {
         if cancel.is_cancelled() {
             return Ok(EvictionCandidates::Cancelled);
         }