Remove array from attestation rewards response

[michaelsproul]

There was an extra layer of unnecessary array nesting in the attestation rewards response:

{
  "data": [
    {"ideal_rewards": [], "total_rewards": []}
  ]
}

The data list should always have 1 element, because the real lists are contained in ideal_rewards and total_rewards. Therefore this structure is more appropriate:

{
  "data": {"ideal_rewards": [], "total_rewards": []}
}

This is already implemented by Lighthouse in sigp/lighthouse#3822.

[rolfyone]

LGTM