optimize indexed attestation validation

[tbenr]

Noticed a potential optimization we can have when we validate IndexedAttestations

• we can preallocate the array to the size we expect
• we can do the sorting check while retrieving the public keys

test run on a very recent mainnet state with an >500 aggregated indexed attestation

407MB vs 1.17GB of Memory allocations

30% faster

Benchmark                                                          Mode  Cnt   Score   Error   Units
IndexedAttestationValidationBenchmark.validateIndexedAttestation  thrpt   25  89.007 ± 1.200  ops/ms

Benchmark                                                          Mode  Cnt    Score   Error   Units
IndexedAttestationValidationBenchmark.validateIndexedAttestation  thrpt   25  116.165 ± 5.730  ops/ms

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.