Use scoped rayon pool for backfill chain segment processing (#7924)

Part of #7866

- Continuation of #7921

In the above PR, we enabled rayon for batch KZG verification in chain segment processing. However, using the global rayon thread pool for backfill is likely to create resource contention with higher-priority beacon processor work.


  This PR introduces a dedicated low-priority rayon thread pool `LOW_PRIORITY_RAYON_POOL` and uses it for processing backfill chain segments.

This prevents backfill KZG verification from using the global rayon thread pool and competing with high-priority beacon processor tasks for CPU resources.

However, this PR by itself doesn't prevent CPU oversubscription because other tasks could still fill up the global rayon thread pool, and having an extra thread pool could make things worse. To address this we need the beacon
processor to coordinate total CPU allocation across all tasks, which is covered in:
- #7789


Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

Co-Authored-By: Eitan Seri- Levi <eserilev@gmail.com>

Co-Authored-By: Eitan Seri-Levi <eserilev@ucsc.edu>

beacon_node/lighthouse_tracing/src/lib.rs

@@ -26,6 +26,7 @@ pub const SPAN_PROCESS_RPC_BLOCK: &str = "process_rpc_block";
 pub const SPAN_PROCESS_RPC_BLOBS: &str = "process_rpc_blobs";
 pub const SPAN_PROCESS_RPC_CUSTODY_COLUMNS: &str = "process_rpc_custody_columns";
 pub const SPAN_PROCESS_CHAIN_SEGMENT: &str = "process_chain_segment";
+pub const SPAN_PROCESS_CHAIN_SEGMENT_BACKFILL: &str = "process_chain_segment_backfill";
 
 /// Fork choice root spans
 pub const SPAN_RECOMPUTE_HEAD: &str = "recompute_head_at_slot";
@@ -61,6 +62,7 @@ pub const LH_BN_ROOT_SPAN_NAMES: &[&str] = &[
     SPAN_PROCESS_RPC_BLOBS,
     SPAN_PROCESS_RPC_CUSTODY_COLUMNS,
     SPAN_PROCESS_CHAIN_SEGMENT,
+    SPAN_PROCESS_CHAIN_SEGMENT_BACKFILL,
     SPAN_HANDLE_BLOCKS_BY_RANGE_REQUEST,
     SPAN_HANDLE_BLOBS_BY_RANGE_REQUEST,
     SPAN_HANDLE_DATA_COLUMNS_BY_RANGE_REQUEST,