Fix edge case where state sync is not triggered (paritytech#5635)

This PR addresses an issue where state sync may fail to start if the
conditions required for its initiation are not met when a finalized
block notification is received. `pending_state_sync_attempt` is
introduced to trigger the state sync later when the conditions are
satisfied.

This issue was spotted when I worked on paritytech#5406, specifically,
`queue_blocks` was not empty when the finalized block notification was
received, and then the state sync was stalled. cc @dmitry-markin

---------

Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Bastian Köcher <git@kchr.de>
(cherry picked from commit 8236718)

prdoc/pr_5635.prdoc

@@ -0,0 +1,13 @@
+title: Fix edge case where state sync is not triggered
+
+doc:
+  - audience: Node Dev
+    description: |
+      There is an edge case where the finalized block notification is received, but the conditions required to initiate the
+      state sync are not fully met. In such cases, state sync would fail to start as expected and remain stalled.
+      This patch addresses it by storing the pending attempt and trying to start the state sync later when the conditions
+      are satisfied.
+
+crates:
+  - name: sc-network-sync
+    bump: patch