Automatically handle both kinds of keystores in "UserWallet.loadSecretKey(), "UserSigner.fromWallet()"

[andreibancioiu]

Automatically handle both kinds of keystore files (with secret key vs. with mnemonic), similar to how it's done in the Python SDK:

• https://github.com/multiversx/mx-sdk-py-wallet/blob/main/multiversx_sdk_wallet/user_wallet.py#L76

Also see:

• https://t.me/MultiversXDevelopers/112354

[andreibancioiu]

Temporary workaround:

const walletObject = JSON.parse(fileContent);
const mnemonic = UserWallet.decryptMnemonic(walletObject, "your keystore password");
const secretKey = mnemonic.deriveKey(0);
const signer = new UserSigner(secretKey);