Add RabbitMQ addon support
Add RabbitMQ as an addon, giving apps access to a managed durable message queue.
This rounds out the initial addon set with a third category (messaging) alongside relational DBs (PostgreSQL, MySQL) and caching/KV (Valkey, Memcache). RabbitMQ is a good fit for the initial set because:
- Simpler to operate than Kafka (single process, no coordination layer)
- Lower resource footprint
- Covers the common use case: background jobs, task queues, basic pub/sub
- Clearly an app-level infrastructure dependency, not a platform concern