Separate BLS key from priv_validator_key.json

[gitferry]

Should consider utilizing some keyring for bls keys storage like ethereum bls key store - https://github.com/ethereum/ercs/blob/master/ERCS/erc-2335.md, or most recent versions of comet bft have support for bls keys - https://github.com/cometbft/cometbft/tree/main/crypto/bls12381

[gitferry]

This should be done in two steps:

1. Naively separate BLS key from priv_validator_key.json as currently it is embeded with comet's ed25519 key for consensus.
2. Store the BLS key securely following https://github.com/ethereum/ercs/blob/master/ERCS/erc-2335.md
3. ensure the BLS key file is created atomically with priv_validator_key.json and remove create-bls-key cmd

[gitferry]

Some context of why and how BLS key is used can be found here.

The BLS key should be treated as important as the ed25519 key used in Comet consens. Currently it is embeded in the priv_validator_key.json and validators should use babylond create-bls-key to generate the BLS key and bond with the ed25519 key generated in pior. However, cosmos tooling (e.g., tmkms and horcrux) cannot recognize the bls key in the priv_validator_key.json, so now we decided to separate it from priv_validator_key.json

In order for secure management of the BLS key, after separation, the BLS key should be kept following standard proposed by Ethereum https://github.com/ethereum/ercs/blob/master/ERCS/erc-2335.md so that it can work with related toolings.