add periodic bootnode ping workflow & add EL genesis files

[pk910]

Added a workflow that periodically checks all CL & EL bootnodes.

This PR also aligns the metadata format with our public devnets (eg. pectra-devnet-2):

• added EL genesis files:
  • metadata/genesis.json reconstructed from geth repo
  • metadata/besu.json copied from besu repo
  • metadata/chainspec.json copied from nethermind repo and removed dead bootnodes
• removed comments from metadata/bootstrap_nodes.txt (not allowed by lighthouse)
• added metadata/bootstrap_nodes.yaml as yaml version of metadata/bootstrap_nodes.txt (with comments)
• added metadata/enodes.txt with a list of EL boot nodes
• added metadata/enodes.yaml as a yaml version of metadata/enodes.yaml

[pk910]

I've tested the EL genesis files with 3 client pairs (geth-lighthouse, besu-lighthouse & nethermind-lighthouse), all of them successfully started syncing the sepolia chain (not using --network sepolia)