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/lighthouse_validator_store/Cargo.toml

@@ -12,6 +12,7 @@ doppelganger_service = { workspace = true }
 either = { workspace = true }
 environment = { workspace = true }
 eth2 = { workspace = true }
+futures = { workspace = true }
 initialized_validators = { workspace = true }
 logging = { workspace = true }
 parking_lot = { workspace = true }