[BUG] Kube-api fetcher interval is hardcoded

**Describe the bug**
The configuration of kube-api fetcher's interval is not being read from the config, but is hardcoded in the code to 30 seconds.
Hardcoded here: https://github.com/elastic/cloudbeat/blob/main/resources/fetchers/kube_fetcher.go#L105

**To Reproduce**
Write the exact actions one should perform in order to reproduce the bug.
Steps to reproduce the behavior:
1. Configure different interval for kube-api fetcher in the config
2. See that it doesn't respect that configured interval

**Expected behavior**
We should read the interval from the config