Store signing keys

Web3Signer supports BLS12-381 (Eth2) or secp256k1 (Eth1) signing keys stored in the following ways:

Key storage	SECP256K1	BLS
Raw files	✅	✅
Keystore files	✅	✅
Vaults
Hashicorp Vault	✅	✅
Azure Key Vault	✅	✅
AWS Secrets Manager		✅
AWS KMS	✅
GCP Secret Manager		✅

Web3Signer supports Eth1 signing from vaults, but must load private keys into memory for Eth2 signing. Follow best practices when storing private keys.

After storing keys, load keys into Web3Signer.