add support for encrypting pkcs12 s/mime key

[tomholub]

Would like to implement this instead of showing error:

  public static encrypt = async (key: Key, passphrase: string) => {
    if (key.type === 'openpgp') {
      return await OpenPGPKey.encryptKey(key, passphrase);
    } else {
      throw new Error(`KeyUtil.encrypt does not support key type ${key.type}`);
    }
  }

The forge library we use likely has a method for this.