ucloud: Callback name improvement.

[sebromero]

The current callback names, on_read and on_write, are confusing for a few reasons:

Ambiguity: These names do not clearly indicate the direction of the data flow or the action being performed. It's not apparent whether the reading or writing is happening on the client-side or the server-side. This ambiguity can lead to misunderstandings about the purpose and functionality of the callbacks.

Lack of context: The names on_read and on_write are generic and could apply to a wide range of scenarios, making it difficult to grasp their specific roles within this particular code. Without a clear context, developers may struggle to understand how these callbacks fit into the overall system.

Terminology inconsistency: The terms "read" and "write" are often associated with file I/O operations, which could lead to confusion when used within a context where data is being sent to and received from the cloud. By using more descriptive names, like on_send_to_cloud and on_update_from_cloud, it becomes easier to differentiate between local file operations and cloud data interactions.

[iabdalkader]

@sebromero I'm closing this now, but feel free to re-open if it's still needed.