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/ExternalSecrets/externalSecretsHelper.ee.ts

@@ -1,6 +1,9 @@
 import { License } from '@/License';
+import config from '@/config';
 import Container from 'typedi';
 
+export const updateIntervalTime = () => config.getEnv('externalSecrets.updateInterval') * 1000;
+
 export function isExternalSecretsEnabled() {
 	const license = Container.get(License);
 	return license.isExternalSecretsEnabled();