Add functionality for storing/recovering `hash-sig` Keypair for Devnet-1

[syjn99]

Describe the feature

Although #792 suggests the endgame (e.g., encryption), I would say we only need a very limited set of functionality for keystore in Devnet-1.

My proposal:

• We want to provide a file that contains "encoded" keypair (Public key + Secret Key).
• Also, as one validator might contain multiple validators, we should b able to provide multiple file paths that contain encoded keypair with a validator index.

Note that SecretKey in hash-sig doesn't have an API like to_public_key().

Additional context

No response

[shariqnaiyer]

@syjn99 I'd love to work on this.