Add methods to set and get custom data for juice agents

[fatduckling]

• Implemented juice_agent_set_data to allow users to attach custom data to a juice agent.
• Implemented juice_agent_get_data to retrieve the custom data attached to a juice agent.
• Updated juice_agent_t structure to include a user_ptr for storing custom data.
• Added tests to validate the functionality of the new methods.

These changes enable each juice agent to have its own custom data, enhancing flexibility and usability.