Improve processAttestations time

### Describe the bug

most of the time to process a block is for `processAttestation`

after electra, a validator could appear in multiple packed attestations, hence we should ignore the 2nd appearance onward

### Expected behavior

process attestation of a validator at most once per block

### Steps to reproduce

_No response_

### Additional context

_No response_

### Operating system

Linux

### Lodestar version or commit hash

v1.36.0

Start at the `processAttestation` processing path identified in the issue and inspect how attestations are handled within a block, especially after Electra. Done means a validator appearing in multiple packed attestations is processed at most once per block.

Written by the indexing model from the issue text.