ZOOKEEPER-4895: Introduce a helper function for C client to generate password for SASL authentication

[empiredan]

C client has provided zoo_sasl_make_basic_callbacks() function to help users initialize SASL callbacks with the password in the specified file. The client would use this password directly for SASL authentication.

However, considering the security of the production environment, the password in a file is usually encrypted. Also, security software also scans files in the system and will issue an alert if it detects an unencrypted password. Therefore, we need a mechanism to read the encrypted text from the file and decrypt it to obtain the real password.

To achieve this, a helper function is introduced to decrypt the encrypted text in the file using a user-provided custom callback. A handback object is also introduced to provide necessary information for the decryption.