Adding the Mekong testnet settings as another supported testnet

[remyroy]

What I did

Added the Mekong testnet settings so it can be used as any other supported testnet with his name without having to manually use the --devnet_chain_setting flag.

You can find and confirm these settings on https://mekong.ethpandaops.io/.

• Genesis fork version: https://github.com/ethpandaops/mekong-devnets/blob/master/network-configs/devnet-0/metadata/config.yaml
• Exit fork version: https://github.com/ethpandaops/mekong-devnets/blob/master/network-configs/devnet-0/metadata/config.yaml
• Genesis validators root: https://github.com/ethpandaops/mekong-devnets/blob/master/network-configs/devnet-0/metadata/genesis_validators_root.txt