https://linear.app/n8n/issue/PAY-933/set-up-leader-selection-for-multiple-main-instances
- [x] Set up new envs
- [x] Add config and license checks
- [x] Implement `MultiMainInstancePublisher`
- [x] Expand `RedisServicePubSubPublisher` to support
`MultiMainInstancePublisher`
- [x] Init `MultiMainInstancePublisher` on startup and destroy on
shutdown
- [x] Add to sandbox plans
- [x] Test manually
Note: This is only for setup - coordinating in reaction to leadership
changes will come in later PRs.