Add support for libzip encryption functions

Originally libZipSharp had no use for libzip's encryption functions as Xamarin.Android doesn't use them, but now that the library is shipped in a nuget those functions may come useful. Implement the following:

- [ ] Explicitly open an encrypted archive
- [ ] Automatically detect if the archive is encrypted and, if yes, use a callback to ask user for password (libZipSharp must not do it on its own)
- [ ] Create an encrypted archive with a provided password and a choice of encryption methods

Relevant upstream APIs:

- [zip_file_set_encryption](https://libzip.org/documentation/zip_file_set_encryption.html)
- [zip_fopen_encrypted](https://libzip.org/documentation/zip_fopen_encrypted.html)
- [zip_fopen_index_encrypted](https://libzip.org/documentation/zip_fopen_index_encrypted.html)
- [zip_set_default_password](https://libzip.org/documentation/zip_set_default_password.html)