Add a new configuration variable CODER_REDIS_URL, that when configured swaps out the following components with Redis versions:

• The API ratelimiter will become distributed using Redis as the backend, rather than being in-memory per-replica
• The pub/sub system will become swapped out with a Redis version, rather than using Postgres LISTEN/NOTIFY
• The tailnet coordinator will become swapped out with a Redis version, rather than using Postgres tables and Postgres LISTEN/NOTIFY

The main benefit would be decreased database load. Redis is very good at handling high throughput for non-persistent data.

Redis support could be gated behind a license feature.