feat: checkpoint-sync-url now can be a list of checkpoint nodes

[LeanSerra]

Motivation

We want the ability to download the state from multiple checkpoint nodes when performing a checkpoint sync then compare the downloaded states failing if any of them are different

Description

• The flag --checkpoint-sync-url now parses a comma separated list of urls then fetch the state from each node finally compare them if any of them is different to the rest the program exits.

Note

• When testing most checkpoint nodes returned error 500 when calling /eth/v1/beacon/deposit_snapshot as this endpoint will be deprecated in the Electra fork. For this reason I didn't add the comparison when downloading the snapshot as it was hard to find two nodes that agreed in the result.

How to try it

rm -rf level_db/mainnet

iex -S mix run -- --checkpoint-sync-url "https://mainnet-checkpoint-sync.stakely.io, https://sync.invis.tools"  --discovery-port 9009 --mode full

Closes #1085

[rodrigo-o]

Just a small comment

[rodrigo-o]

I'm not fan of Halts, but given that it was already used in the previous state of the function we could just maintain it here, in any case i think it's better to go for the positive and halt in the else instead of simulating and early return.

    if anchor_state.latest_block_header.parent_root == first_block.message.parent_root do
      {:ok, store} = Store.get_forkchoice_store(anchor_state, anchor_block)
  
      # Save store in DB
      StoreDb.persist_store(store)
  
      store
    else
      Logger.error(
        "[Checkpoint sync] Root mismatch when comparing latest finalized block with downloaded state"
      )

      Logger.flush()
      System.halt(1)
    end