feat: support loading private key

[malm0d]

To load using private key, we can now use the --private-key option. And if we want to load with private key using flashbots send bundle mode, we can pass another option --flashbots-private-key. An addition option is required for using flashbots mode while loading with private key, since the private key only maps to a single address without hierarchical derivation.

Sample command to load using private key:
cargo run --release -- node --eth-client-address http://127.0.0.1:8545 --private-key 4c5e5d30… --beneficiary 0xf39Fd6e5… --entry-points 0x5FF137D4… --http --ws

Sample command to load using private key with flashbots mode:
cargo run --release -- node --eth-client-address http://127.0.0.1:8545 --private-key 4c5e5d30… --flashbots-private-key df218be... --send-bundle-mode flashbots --beneficiary 0xf39Fd6e5… --entry-points 0x5FF137D4… --http --ws

• The --mnemonic-file and --private-key options are mutually exclusive, one and only one can be used.
• Similarly, --flashbots-private-key cannot be used in the same command as --mnemonic-file, and it cannot be used alone without the --private-key option; but it can be optional when --private-key is used.
• If --send-bundle-mode is not flashbots and --flashbots-private-key is provided, --flashbots-private-key will be ignored.

Additional considerations:
I added a value-parser for the --private-key and --flashbot-private-key arguments, and the current implementation only checks if there are 64 characters and if each char is an ASCII hexadecimal digit. Am wondering if this is sufficient or if theres another better way to do this:

pub fn validate_private_key(hex_string: &str) -> Result<String, String> {
    let chars = hex_string.chars();

    if chars.clone().count() != 64 {
        return Err(format!("{hex_string} is not a valid private key"));
    }

    for c in chars {
        if !c.is_ascii_hexdigit() {
            return Err(format!("{hex_string} is not a valid hexadecimal string"));
        }
    }

    Ok(String::from(hex_string))
}

[Vid201]

This PR looks really good and clean, thanks!

I would just remove the check validate_private_key because the program would terminate either way if there is a wrong format/key - when converting to wallet in wallet.rs.

[Vid201]

Merging this now, thanks for contributing!!