feat: electra get_attesting_indices changes + helper functions from predicates/misc.

[LeanSerra]

Motivation

Implement the changes for Electra for sections predicates, misc

This PR does not fix any spec-test but does not break any either to check this

make spec-test

should return same amount of tests as #1417

Description
Predicates

• Modified is_eligible_for_activation_queue link
• New is_compounding_withdrawal_credential link
• New has_compounding_withdrawal_credential link
• New has_execution_withdrawal_credential link
• Modified is_fully_withdrawable_validator link
• Modified is_partially_withdrawable_validator link

Misc

• New get_committee_indices link

Beacon state accessors

• Modified get_attesting_indices link

[rodrigo-o]

LGTM, just a comment that could cause issues in the future (in the get_attesting_indices) and another one to show how comprehension could match closer the spec if needed.

[rodrigo-o]

We may have an issue here, before when the get_beacon_committee/3 returned an error we just bubbled it up, now if this happens the reduce will fail because it will try to parse an {:error, error} as the {old_set, offset} tuple. We could go with a reduce_while, instead (BTW made some changes for readability):

  committee_bits
  |> get_committee_indices()
  |> Enum.reduce_while({MapSet.new(), 0}, fn committee_index, {attesters, offset} ->
    case get_beacon_committee(state, data.slot, committee_index) do
      {:ok, committee} ->
        # Process this committee's attesters
        committee_attesters = 
          committee
          |> Stream.with_index(offset)
          |> Stream.filter(fn {_validator, pos} -> participated?(aggregation_bits, pos) end)
          |> Stream.map(fn {validator, _} -> validator end)
          |> MapSet.new()
        
        {:cont, {MapSet.union(attesters, committee_attesters), offset + length(committee)}}
      
      error -> 
        # Depending on the specs we might just want to skip this error with something like: `{attesters, offset}`
        {:halt, error}
    end
  end)
  |> case do
    {:error, error} -> {:error, error}
    {attesters, _offset} -> {:ok, attesters}
  end

Also, I'm not 100% sure about the usefulness of Streams here, but given that it was already implemented that way I wouldn't change it to Enum for now.

[LeanSerra]

Ooops I didn't know with behaved that way. I implemented the changes here. I also tried both {:halt, error} and {:cont, {attesters, offset} and the amount of tests passing is the same so for now I think we can leave it as {:halt, error}.