Support keystore and password file path flags in es-node

[qzhodl]

Rationale

Why should this feature exist?
Prysm have similar options like --wallet-dir and --wallet-password-file, and the related references are https://docs.prylabs.network/docs/prysm-usage/parameters and https://docs.prylabs.network/docs/wallet/deterministic

Implementation

Do you have ideas regarding the implementation of this feature?
Are you willing to implement this feature?

[syntrust]

Optimism:

1. a remote signer (via opsigner.CLIConfig) or
2. a mnemonic + derivation path or
3. a private key.

reference:
https://github.com/ethereum-optimism/optimism/blob/be68885e45edda08deec735d1995f3500ad35b1b/op-service/crypto/signature.go#L46

Prysm:

1. external-signer: URL for consensys' web3signer software to use with the Prysm validator client.
2. wallet-dir + wallet-password-file

reference:
https://docs.prylabs.network/docs/prysm-usage/parameters#validator-flags

[qzhodl]

Since Optimism doesn’t support the keystore either, we can deprioritize this issue.