Electra: Handle AttestationElectra in forkchoice spectest

[prestonvanloon]

What type of PR is this?

Other

What does this PR do? Why is it needed?

This still fails, but no longer failing for ssz unmarshalling errors.
The test is currently skipped, but unskipping shows the following errors for minimal spectest

--- FAIL: TestMinimal_Electra_Forkchoice (26.47s)                                                                                                                         
    --- FAIL: TestMinimal_Electra_Forkchoice/voting_source_within_two_epoch (0.60s)                                                                                       
        assertions.go:144: builder.go:139 Unexpected error: could not create attestation ID: 2 committee bits are set instead of 1                                        
    --- FAIL: TestMinimal_Electra_Forkchoice/justification_withholding_reverse_order (0.39s)                                                                              
        assertions.go:144: builder.go:139 Unexpected error: could not create attestation ID: 2 committee bits are set instead of 1                                        
    --- FAIL: TestMinimal_Electra_Forkchoice/simple_blob_data (0.08s)                                                                                                     
        runner.go:386: SidecarInclusionProven error: sidecar inclusion proof verification failed: blob failed verification                                                
        runner.go:431: sidecar inclusion proof verification failed: blob failed verification - RequireSidecarInclusionProven                                              
    --- FAIL: TestMinimal_Electra_Forkchoice/withholding_attack_unviable_honest_chain (0.36s)                                                                             
        assertions.go:144: builder.go:139 Unexpected error: could not create attestation ID: 2 committee bits are set instead of 1                                        
FAIL

Which issues(s) does this PR fix?

Other notes for review