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.