PgMq Backend for Apalis

[nutcas3]

cc @geofmureithi how is this provide feedback on this then

Overview

This PR introduces a new PostgreSQL-based message queue backend for the Apalis job processing library using the pgmq crate. It provides a robust, persistent queue implementation for background job processing.

Features

• PgMq Backend: Full implementation of apalis_core::backend::Backend trait
• Task Management: Support for enqueuing, polling, and acknowledging tasks
• Message Handling: Automatic message archival after max retries
• Configurable Parameters:
  • Visibility timeout for message locks
  • Custom poll intervals
  • Configurable max retry attempts
• Context Tracking: Includes message metadata (ID, read count, timestamps) in task context

Implementation Details

• Uses pgmq for PostgreSQL-based message queue operations
• Implements proper error handling with custom error types
• Follows Rust best practices with proper module organization
• Includes comprehensive documentation

Usage Example

let pgmq = PgMq::new("postgres://user:pass@localhost:5432/db", "my_queue").await?;
pgmq.enqueue(MyJob { /* ... */ }).await?;