Resolve Panic in ListValidatorBalances RPC

[rauljordan]

This resolves #4020

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Description

Write why you are making the changes in this pull request

This PR resolves a bug in in ListValidatorBalances that would lead to a panic at runtime when using archival functionality:

returns rpc error: code = Unknown desc = runtime error: index out of range [345] with length 345

Specifically, this would happen under the following conditions:

1. Current epoch is 100
2. Request balances for epoch 50
3. More validators exist in the state at epoch 100 than at epoch 50
4. See the code below:

    // return everything.
    for i := 0; i < len(headState.Balances); i++ {
        res = append(res, &ethpb.ValidatorBalances_Balance{
            PublicKey: headState.Validators[i].PublicKey,
            Index:     uint64(i),
            Balance:   archivedBalances[i],
        })
    }

5. We would loop over headState.Balances, which might have len > archivedBalances at epoch 50
6. We would index into archivedBalances[i], which would cause a panic

Write a summary of the changes you are making

The loop should be over the archived balances, not over headState.Balances. A regression test was added to ensure the proper default responses for both archival and regular functionality so this will never happen again.

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[shayzluf]

lgtm

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