feat: Add config option for external secret update interval (#7995)

## Summary
Adds `N8N_EXTERNAL_SECRETS_UPDATE_INTERVAL` to allow enterprise users to
tweak the update internal for importing new secrets.

If using a config file the value is:

```
"externalSecrets": {
  "updateInterval": 300
}
```

#### How to test the change:
1. Run as normal and check that the secret is updated every 5 minutes
2. Set `N8N_EXTERNAL_SECRETS_UPDATE_INTERVAL` to 10
3. Check the secret is reloaded after 10 seconds


## Review / Merge checklist
- [x] PR title and summary are descriptive. **Remember, the title
automatically goes into the changelog. Use `(no-changelog)` otherwise.**
([conventions](https://github.com/n8n-io/n8n/blob/master/.github/pull_request_title_conventions.md))
- [x] [Docs updated](https://github.com/n8n-io/n8n-docs) or follow-up
ticket created.

packages/cli/src/config/schema.ts

@@ -1007,6 +1007,15 @@ export const schema = {
 		},
 	},
 
+	externalSecrets: {
+		updateInterval: {
+			format: Number,
+			default: 300,
+			env: 'N8N_EXTERNAL_SECRETS_UPDATE_INTERVAL',
+			doc: 'How often (in seconds) to check for secret updates.',
+		},
+	},
+
 	deployment: {
 		type: {
 			format: String,