createPushEx abstract method

Provides crypto_secretstream_xchacha20poly1305_(init_)push.

Works just like pushEx, but instead of creating a stream, it creates a SecretExStreamTransformer with key, which can then be used to create push encryption streams from it.

See https://libsodium.gitbook.io/doc/secret-key_cryptography/secretstream#encryption

Implementation

SecretExStreamTransformer<SecretStreamPlainMessage, SecretStreamCipherMessage>
    createPushEx(SecureKey key);