Add MongoDB change streams support

[anitnilay20]

Description

Implement MongoDB change streams for real-time data monitoring and reactive applications.

Requirements

• Change stream subscription
• Filter change events
• Resume token handling
• Event type handling (insert, update, delete, replace)
• Collection and database-level streams
• Write comprehensive tests

Acceptance Criteria

• Real-time change monitoring
• Event filtering capabilities
• Reliable stream resumption
• Type-safe event handling

Files to create

• beatrix_core/src/mongo/change_streams.rs

Example Usage

let mut stream = User::watch()
    .filter(doc! { "operationType": "insert" })
    .start(&db).await?;

while let Some(event) = stream.next().await {
    match event {
        ChangeEvent::Insert { document, .. } => {
            println!("New user: {:?}", document);
        }
        _ => {}
    }
}