Add sign_voluntary_exit_message command

[lean-apple]

Add sign-voluntary-exit command to be used like this :

cargo run --bin coral-cli validator sign-voluntary-exit \
  --bls-public-key 0x94505f60bb8e48ddafb8835ec08537c48ed1bb9bc6a95fe941f37869b5eb0950c9023b7a997fe12d8aa79076561e009f \
  --beacon-index 1605300 \
  --enclave-url http://localhost:9001 \
  --fork-previous-version 0x04017000 \
  --fork-current-version 0x05017000 \
  --epoch 29696 \
  --genesis-validators-root 0x9143aa7c615a7f7115e2b6aac319c03529df8242ae705fba9df39b79c59fa8b1 \
  --output-file sign_vem_001.json

File output format :

{
  "signature": "0xb3ace343a1fa578429d171526d2a318298b00d5b4dca4212aac374b61eb8436ccdb363c823fb3c75b504cc17a023b00808d6cad733215c39da71339f49047e103df7bf072fc0fbf4f5cda3a9fa21c03a68a93326669bb657e8a1af22bff00670",
  "beacon_index": 1605300,
  "epoch": 29696,
  "bls_pubkey": "0x94505f60bb8e48ddafb8835ec08537c48ed1bb9bc6a95fe941f37869b5eb0950c9023b7a997fe12d8aa79076561e009f"
}

yet @kamiyaa just wondering if we keep the beacon_client part in this new coral,
Here we just use it to get beacon_index, I don't think it's so annoying to push as an argument in the command instead

• Remove all beacon part and move beacon_index as a command arg
• Test with validator_enclave
• Docs

Next step : integrate to broadcast in front-end