Refactor keystore to get rid of additional decrypt

[mksh]

• Add VotingKeyMaterial wrapper struct to use in place of Lighthouse's Keystore, which carries plain text private key of a validator
• Get rid of additional keystore .decrypt() call on export, using private key of VotingKeyMaterial
• Thus, remove 30% of call length, now 1 validator is generated in less than 10 seconds on my Mac

[jenpaff]

• Add VotingKeyMaterial wrapper struct to use in place of Lighthouse's Keystore, which carries plain text private key of a validator

  • Get rid of additional keystore .decrypt() call on export, using private key of VotingKeyMaterial

  • Thus, remove 30% of call length, now 1 validator is generated in less than 10 seconds on my Mac

almost means we can remove pregeneration