Check slashability of attestations in batches to avoid sequential bottleneck (#8516)

Closes:

- https://github.com/sigp/lighthouse/issues/1914


  Sign attestations prior to checking them against the slashing protection DB. This allows us to avoid the sequential DB checks which are observed in traces here:

- https://github.com/sigp/lighthouse/pull/8508#discussion_r2576686107


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

Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

common/task_executor/src/rayon_pool_provider.rs

@@ -15,7 +15,7 @@ pub struct RayonPoolProvider {
     /// By default ~25% of CPUs or a minimum of 1 thread.
     low_priority_thread_pool: Arc<ThreadPool>,
     /// Larger rayon thread pool for high-priority, compute-intensive tasks.
-    /// By default ~80% of CPUs or a minimum of 1 thread. Citical/highest
+    /// By default ~80% of CPUs or a minimum of 1 thread. Critical/highest
     /// priority tasks should use the global pool instead.
     high_priority_thread_pool: Arc<ThreadPool>,
 }