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>

validator_client/slashing_protection/src/lib.rs

@@ -16,8 +16,8 @@ pub mod interchange {
 pub use crate::signed_attestation::{InvalidAttestation, SignedAttestation};
 pub use crate::signed_block::{InvalidBlock, SignedBlock};
 pub use crate::slashing_database::{
-    InterchangeError, InterchangeImportOutcome, SUPPORTED_INTERCHANGE_FORMAT_VERSION,
-    SlashingDatabase,
+    CheckSlashability, InterchangeError, InterchangeImportOutcome,
+    SUPPORTED_INTERCHANGE_FORMAT_VERSION, SlashingDatabase,
 };
 use bls::PublicKeyBytes;
 use rusqlite::Error as SQLError;