feat(core): Set up leader selection for multiple main instances (#7527)

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.

packages/cli/src/services/orchestration/main/handleCommandMessageMain.ts

@@ -34,7 +34,8 @@ export async function handleCommandMessageMain(messageString: string) {
 					};
 					return message;
 				}
-				if (isMainInstance) {
+
+				if (isMainInstance && !config.getEnv('leaderSelection.enabled')) {
 					// at this point in time, only a single main instance is supported, thus this command _should_ never be caught currently
 					logger.error(
 						'Received command to reload license via Redis, but this should not have happened and is not supported on the main instance yet.',