Add support for EIP-2335 keystores

[MegaRedHand]

Is your feature request related to a problem? Please describe.

I was trying to generate keys with eth2-val-tools for the Incredible Squaring example, and it seems to fail because it can't parse the generated keystone. This happens at crypto/bls/attestation.go, inside ReadPrivateKeyFromFile.

Describe the solution you'd like

I'd like the operator to accept EIP-2335 keystores.

Describe alternatives you've considered

I could use egnkey to generate the keystone, but it doesn't support as many options.

Additional context

[shrimalmadhur]

thanks for raising this - we are planning to integrate this soon - https://github.com/Layr-Labs/bn254-keystore-go for bn254 keys, which is used across the board.

Will keep it backward compatible.